Welcome to the forum!
Here are some pictures of the accessory, including a view of the bottom.
http://www.tandem-associates.com/lionel/lionel_trains_1045_acc.htmThere seems to be some insulating washers under the mounting nuts.
Your suggestion about getting some plastic washers is a good one. More than likely the original insulators may have been fiber.
I am surprised the accessory came the way you describe. Did you get it used?
Larry