When I need a dict like field in a script in any of packages ,I use a struct with two fields. This is a simple drawer for that.

SimpleTypeValueDrawer.cs

using UnityEditor;
using UnityEngine;

namespace ubco.ovilab.ViconUnityStream.Editor
{
    [CanEditMultipleObjects]
    [CustomPropertyDrawer(typeof(S), true)]
    public class SimpleTypeValueDrawer: PropertyDrawer
    {
        public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
        {
            return EditorGUIUtility.singleLineHeight + EditorGUIUtility.standardVerticalSpacing;
        }

                public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
        {
            EditorGUI.BeginProperty(position, label, property);
            SerializedProperty keyProp = property.FindPropertyRelative("key");
            SerializedProperty valueProp = property.FindPropertyRelative("value");

            float width = EditorGUIUtility.currentViewWidth;
            float valueHeight = EditorGUIUtility.singleLineHeight;

            // Calculate rects
            Rect keyRect = new Rect(position.x, position.y, position.width * 0.5f, valueHeight);
            Rect valueRect = new Rect(position.x + position.width * 0.51f, position.y, position.width * 0.5f, valueHeight);

            // Draw fields - pass GUIContent.none to each so they are drawn without labels
            EditorGUI.PropertyField(keyRect, keyProp, GUIContent.none);
            EditorGUI.PropertyField(valueRect, valueProp, GUIContent.none);

            EditorGUI.EndProperty();
        }
    }
}