I have been using the Pink foam you can buy in sheets at Home Depot. Not all stores have it but my local one here have 2" thick sheets. Some only have the 1/2" sheets or maybe they are 5/8".
It cuts very clean and is stiff enough that you can clamp it.
Todd, Steen,
Thanks to you both for the help. My local Home depot only sells the thin stuff, but I'll make some phone calls.
I appreciate the suggestions very much.
Paul