Put an even coat of Johnson's paste wax on the bottom and the edges of the roadbed , and let it dry before you lay it on the joint compound or other sculpture product. (Use the type of wax that comes in a can.) I would doubt very much if the compound will stick.
Gunsmiths use this method to protect the metal barrel and action of a rifle before they glass bed the inside of a stock and set the action down into the glass bedding gel. (Often they are dealing with an extremely expensive rifle.) Once the bedding dries, the action and barrel and be easily pulled off of the gel.
Just don't let the compound slop up over the edges of the roadbed when you press it down.
Hope this helps,
Mannyrock
