CustomToggle

public bool CustomToggle(bool myToggle, int height, int width) {
    string boxChecked;

    if (myToggle) {
        boxChecked = "X";
    } else {
        boxChecked = "";
    }

    if (GUILayout.Button(boxChecked, GUILayout.Height(height),
        GUILayout.Width(width))) {

        myToggle = !myToggle;
    }

    return myToggle;
}

public override void OnInspectorGUI()
{
    EditorGUIUtility.LookLikeInspector();
    EditorGUILayout.BeginVertical();

    for (int y = 0; y < Target.MaskHeight; y++)
    {
        EditorGUILayout.BeginHorizontal();
        for (int x = 0; x < Target.MaskWidth; x++)
        {
            Target.Mask[x,y] = CustomToggle(Target.Mask[x, y], 16, 16);
        }
        EditorGUILayout.EndHorizontal();
    }

    EditorGUILayout.EndVertical();
}