Dental Dams? LOL. "No, honestly, I want to push coins through them."
I'm not too sure about letting a spectator do it. Not so much because they might work it out, as for an increased risk of them tearing the sheet (too much force, or long nails).
Mind you, we all know how good it is to let the spectator work magic in his or her own hands, so the trade-off might be good.
When I first bought this (called "Gozinta and produced by the Inzani-Henley Magic Compnay), it came with one of those "gimmicks".
There is another advantage to using a silver coin. They have milled edges. 'Nuff said.
I got a version some years ago cast in green latex, with four counters (like tiddly winks). You did a "Magician's Choice" force. The counters were good because they are colourful, but bad because they were so smooth and, I suppose, because they are not quite as commonplace as coinage!