The best way to do it withresins is with our Waterclear Polyurethane Casting Resin.  You will need to do the pouring in a dry andwarm room preferably over 25C and pour at least a minimum of 2mm thick over thetop level of any coins, preferably a bit thicker.

You will need to put some kind of dust shieldover the top to ensure no dust or bugs land in the resin while it cures.

The far cheaper way I suspect would be a sheet oftoughened glass over the top, although that may not be what the client wants.