I'm bumping this up because I may have figured this out myself
What if I create a background on 4x6 cardstock and cut little slits at the ends where the pennies can slide into. Then slide the card into a small photo album. It'd be small enough to carry around the park if we so desired though slightly labor intensive to put the pennies in (not too bad, but more than can be done while standing in from of the penny machine).
WDYT?
Of course whether that will actually get done has yet to be determined.....too many ideas floating around in my brain....so little time.