Depth Cube

BorderedDepthCube.shader

Shader "Unlit/BorderedDepthCube"
{
	Properties
	{
		_Color("Colour", Color) = (1,1,1,1)
		_BorderWidth("Border Width", float) = 0.03
		_BorderStrength("Border Strength", float) = 0.3
	}
	SubShader
	{
		Tags { 
			"RenderType"="Transparent"
			"Queue" = "Transparent"
			"DisableBatching" = "True"
		}
		LOD 100
		Blend SrcAlpha OneMinusSrcAlpha
		

		Cull Back
		ZWrite Off

		CGINCLUDE
			#pragma vertex vert
			#pragma fragment frag

			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex : POSITION;
				float2 uv : TEXCOORD0;
				float4 normal : NORMAL;
				float4 tangent : TANGENT;
			};

			struct v2f
			{
				float4 vertex : SV_POSITION;
				float2 uv : TEXCOORD0;
				float3 viewDir : TEXCOORD1;
			};

			float4 _Color;
			float _BorderWidth;
			float _BorderStrength;
			
		ENDCG

		Pass
		{
			CGPROGRAM
			v2f vert (appdata v)
			{
				v2f o;
				o.vertex = UnityObjectToClipPos(v.vertex);

				float3 binormal = cross(v.normal, v.tangent.xyz) * v.tangent.w;
				float3 viewDirInObjectCoords = mul(
					unity_WorldToObject, float4(_WorldSpaceCameraPos, 1.0)).xyz
					- v.vertex.xyz;
				float3x3 localSurface2ScaledObjectT =
					float3x3(v.tangent.xyz, binormal, v.normal.xyz);
				float3 viewDirT = mul(localSurface2ScaledObjectT, viewDirInObjectCoords);
				o.viewDir = viewDirT;
				o.uv = v.uv;

				return o;
			}
			
			fixed4 frag (v2f i) : SV_Target
			{	
				fixed3 viewDir = normalize(i.viewDir);


				float distToGround = max(length((i.uv.y / i.viewDir.y)* i.viewDir), (i.viewDir.y<0)*3);
				float distToRoof = max(length((-(1.0f - i.uv.y) / i.viewDir.y)* i.viewDir), (i.viewDir.y>0)*3);
				float distToLeft = max(length(((i.uv.x ) / i.viewDir.x) * i.viewDir), (i.viewDir.x<0)*3);
				float distToRight = max(length((-(1.0f - i.uv.x ) / i.viewDir.x) * i.viewDir), (i.viewDir.x>0)*3);
				float distToBack = length(1 / i.viewDir.z * i.viewDir);

				float minDist = min(distToBack, min( min(distToGround, distToRoof), min(distToLeft, distToRight)));
				float2 twoBorders = 1 - abs(i.uv - 0.5) * 2;
				float border = (1-smoothstep(0, _BorderWidth, min(twoBorders.x,twoBorders.y)));
				border = smoothstep(0,minDist, _BorderWidth) * _BorderStrength;
				minDist += border;
				return fixed4(_Color.rgb, _Color.a * clamp(0,1,minDist));

			}
			ENDCG
		}
	}
	FallBack "Particles/AlphaBlended"
}

DepthCube.shader

Shader "Unlit/DepthCube"
{
	Properties
	{
		_Color("Colour", Color) = (1,1,1,1)
	}
	SubShader
	{
		Tags { 
			"RenderType"="Transparent"
			"Queue" = "Transparent"
			"DisableBatching" = "True"
		}
		LOD 100
		Blend SrcAlpha OneMinusSrcAlpha
		

		Cull Back
		ZWrite Off

		CGINCLUDE
			#pragma vertex vert
			#pragma fragment frag

			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex : POSITION;
				float2 uv : TEXCOORD0;
				float4 normal : NORMAL;
				float4 tangent : TANGENT;
			};

			struct v2f
			{
				float4 vertex : SV_POSITION;
				float2 uv : TEXCOORD0;
				float3 viewDir : TEXCOORD1;
			};

			float4 _Color;
			
		ENDCG

		Pass
		{
			CGPROGRAM
			v2f vert (appdata v)
			{
				v2f o;
				o.vertex = UnityObjectToClipPos(v.vertex);

				float3 binormal = cross(v.normal, v.tangent.xyz) * v.tangent.w;
				float3 viewDirInObjectCoords = mul(
					unity_WorldToObject, float4(_WorldSpaceCameraPos, 1.0)).xyz
					- v.vertex.xyz;
				float3x3 localSurface2ScaledObjectT =
					float3x3(v.tangent.xyz, binormal, v.normal.xyz);
				float3 viewDirT = mul(localSurface2ScaledObjectT, viewDirInObjectCoords);
				o.viewDir = viewDirT;
				o.uv = v.uv;

				return o;
			}
			
			fixed4 frag (v2f i) : SV_Target
			{	
				fixed3 viewDir = normalize(i.viewDir);

				float distToGround = max(length((i.uv.y / i.viewDir.y)* i.viewDir), (i.viewDir.y<0)*3);
				float distToRoof = max(length((-(1.0f - i.uv.y) / i.viewDir.y)* i.viewDir), (i.viewDir.y>0)*3);
				float distToLeft = max(length(((i.uv.x ) / i.viewDir.x) * i.viewDir), (i.viewDir.x<0)*3);
				float distToRight = max(length((-(1.0f - i.uv.x ) / i.viewDir.x) * i.viewDir), (i.viewDir.x>0)*3);
				float distToBack = length(1 / i.viewDir.z * i.viewDir);

				float minDist = min(distToBack, min( min(distToGround, distToRoof), min(distToLeft, distToRight)));

				return fixed4(_Color.rgb, _Color.a * clamp(0,1,minDist));

			}
			ENDCG
		}
	}
	FallBack "Particles/AlphaBlended"
}

IconDepthCube.shader

Shader "Unlit/IconDepthCube"
{
	Properties
	{
		_Color("Colour", Color) = (1,1,1,1)
		_MainTex("Icon Texture", 2D) = "white" {}
		_IconDepth("Icon Depth", float) = 0.5
	}
	SubShader
	{
		Tags { 
			"RenderType"="Transparent"
			"Queue" = "Transparent"
			"DisableBatching" = "True"
		}
		LOD 100
		Blend SrcAlpha OneMinusSrcAlpha
		

		Cull Back
		ZWrite Off

		CGINCLUDE
			#pragma vertex vert
			#pragma fragment frag

			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex : POSITION;
				float2 uv : TEXCOORD0;
				float4 normal : NORMAL;
				float4 tangent : TANGENT;
			};

			struct v2f
			{
				float4 vertex : SV_POSITION;
				float2 uv : TEXCOORD0;
				float3 viewDir : TEXCOORD1;
			};

			float4 _Color;
			sampler2D _MainTex;
			float4 _MainTex_ST;
			float _IconDepth;
			
		ENDCG

		Pass
		{
			CGPROGRAM
			v2f vert (appdata v)
			{
				v2f o;
				o.vertex = UnityObjectToClipPos(v.vertex);

				float3 binormal = cross(v.normal, v.tangent.xyz) * v.tangent.w;
				float3 viewDirInObjectCoords = mul(
					unity_WorldToObject, float4(_WorldSpaceCameraPos, 1.0)).xyz
					- v.vertex.xyz;
				float3x3 localSurface2ScaledObjectT =
					float3x3(v.tangent.xyz, binormal, v.normal.xyz);
				float3 viewDirT = mul(localSurface2ScaledObjectT, viewDirInObjectCoords);
				o.viewDir = viewDirT;
				o.uv = v.uv;

				return o;
			}
			
			fixed4 frag (v2f i) : SV_Target
			{	
				fixed3 viewDir = normalize(i.viewDir);

				float distToGround = max(length((i.uv.y / i.viewDir.y)* i.viewDir), (i.viewDir.y<0)*3);
				float distToRoof = max(length((-(1.0f - i.uv.y) / i.viewDir.y)* i.viewDir), (i.viewDir.y>0)*3);
				float distToLeft = max(length(((i.uv.x ) / i.viewDir.x) * i.viewDir), (i.viewDir.x<0)*3);
				float distToRight = max(length((-(1.0f - i.uv.x ) / i.viewDir.x) * i.viewDir), (i.viewDir.x>0)*3);
				float distToBack = length(1 / i.viewDir.z * i.viewDir);
				float4 centreTexture = tex2D(_MainTex, clamp(i.uv - i.viewDir.xy/i.viewDir.z * _IconDepth, 0,1));

				float minDist = min(distToBack, min( min(distToGround, distToRoof), min(distToLeft, distToRight)));
				minDist = lerp(minDist, centreTexture.r, centreTexture.a);
				return fixed4(_Color.rgb, _Color.a * minDist);

			}
			ENDCG
		}
	}
	FallBack "Particles/AlphaBlended"
}

It's only sure to work on cubes and flat surfaces. It assumes a 0 to 1 uv map represents the face of a cube and calcs the distance to the other five faces. If you have something intersect the surface it will look wrong because it will still show the depth of the back of the cube rather than the depth of the intersecting object.

If you scale it up or down, the distance to the other side of the cube scales appropriately, but the opacity is always the same for one cube's length, no matter how large that cube is. So a large cube will seem like a lower density.

Maybe you could do a cool gem thing with clever manipulation of the normals.