RE: Help for making a plug from foam
I am not sure by your post if you are making a male plug to use to make a female mold or if you are going to try to make a finished part off the foam plug.
If you are going to make a female mold off the plug, do as Butch says. Lay up the plug with epoxy resing and glass cloth. The glass cloth needs to be fairly thick (stiff) so it doesn't flex too much when sanding the glass. I recently make some tank molds and only used two layers of 3.2oz cloth and the foam would flex when sanding and caused me a lot of work to get a good finish.