RenderEquirectWizard.cs

// From https://docs.unity3d.com/2018.1/Documentation/ScriptReference/Camera.RenderToCubemap.html
using UnityEngine;
using UnityEditor;
using UnityEngine.Rendering;
using System.Collections;

public class RenderEquirectWizard : ScriptableWizard
{
    public Transform renderFromPosition;
    RenderTexture cubemap;
    public RenderTexture equirect;

    private void OnEnable()
    {
        cubemap = new RenderTexture(8192, 8192, 24, RenderTextureFormat.ARGB32);
        cubemap.dimension = UnityEngine.Rendering.TextureDimension.Cube;
        equirect = new RenderTexture(4096, 2048, 24, RenderTextureFormat.ARGB32);
        cubemap.Create();
        equirect.Create();
    }

    void OnWizardUpdate()
    {
        string helpString = "Select transform to render from and cubemap to render into";
        bool isValid = (renderFromPosition != null) && (cubemap != null);
    }

    void OnWizardCreate()
    {
        // create temporary camera for rendering
        GameObject go = new GameObject("CubemapCamera");
        go.AddComponent<Camera>();
        // place it on the object
        go.transform.position = renderFromPosition.position;
        go.transform.rotation = Quaternion.identity;
        // render into cubemap
        go.GetComponent<Camera>().RenderToCubemap(cubemap);
        cubemap.ConvertToEquirect(equirect, Camera.MonoOrStereoscopicEye.Mono);


        byte[] bytes = toTexture2D(equirect).EncodeToPNG();
        System.IO.File.WriteAllBytes(Application.dataPath + "/equirect.png", bytes);
       

        //Output the Game data path to the console
        Debug.Log("Path : " + Application.dataPath);

        // destroy temporary camera
        DestroyImmediate(go);
    }

    [MenuItem("GameObject/Render Equirect")]
    static void RenderCubemap()
    {
        ScriptableWizard.DisplayWizard<RenderEquirectWizard>(
            "Render Equirect", "Render!");
    }

    Texture2D toTexture2D(RenderTexture rTex)
    {
        Texture2D tex = new Texture2D(4096, 2048, TextureFormat.RGB24, false);
        RenderTexture.active = rTex;
        tex.ReadPixels(new Rect(0, 0, rTex.width, rTex.height), 0, 0);
        tex.Apply();
        return tex;
    }
}