How to create a new SerializedProperty?

Recently Custom Editors in Unity caught my attention. Now I'm having a small problem.

I'm currently making a custom editor for a script with 5 properties. The first property is a color value, and will always be visible. The folowing enum should also be visible:

 private enum MaterialType { Color, Pattern }

The other four properties should show up when you select the "Pattern" value. I'm using this code to show my enum:

     MaterialType materialType = MaterialType.Color;
     EditorGUILayout.PropertyField(materialType, new GUIContent("Color or pattern?"));

I'm getting this error: Cannot convert type 'TileObjectEditor.MaterialType' to 'UnityEditor.SerializedProperty'. I understand this error message, and my question is how to get the enum value into a SerializedProperty.

Note that the enum is not included in the base script itself. It is only needed in the editor script, so I can't use serObj.findProperty(...). I could solve this all by moving the enum to the base script, and just use serObj.findProperty("MaterialType"), but since MaterialType is unrelevant there, I'm looking for another way.