Bill, I would imagine that would be no problem, you would need perhaps to experiment with mold release, that is, which method would give best results, water with detergent, silicone spray, etc. Once that problem is solved, apply foam to mold and flip over onto flat surface, maybe tape it down till set. About 25 years ago I used "Mountains in Minutes", perhaps still available, a 2 part liquid which when combined expanded to something like 20 times its volume and resulted in a smooth skinned foam. As I recall, it could be used for molds as you asked about. I didn't use it that way, I was building a module to house a Campbell bridge I had built(just to see if I could). I had laid out the stream bed and wanted to use the foam for the banks and surrounding land. I underestimated the expansion and had to work frantically to pull the expanding foam back from the bank area. When you do this, the foam is pretty sticky, and starting to set up, so you get some interesting land forms! Mostly it worked out fine, those areas which looked unnatural I simply carved, when you do so you can forget about that smooth skin, underneath the air bubbles are significant. I carved a large area flat, filled the holes with spackle and after painting had a large flat shale type rock as seen along the Delaware river. Sometimes having no time to think can result in an effect you wouldn't have otherwise thought of. Then again, sometimes it doesn't!
Gary
