Aluminum foil pizza pans, those throw-aways: just use a rolling pin to flatten the raised edge and cut the center out as Deb says. Then use E6000 glue for the pennies and it will last forever. The foam board that Becky mentioned would work too, but might break as foam board often does when stepped on. I'm notorious for stepping in the wrong place in my gardens. Anyway, seems like those pizza pans that are like 2 for a dollar at any dollar store might last as long as the pennies do. Elmer's glue is water soluble, but the E6000 works well for any outside crafts.
Great idea!!
