Just make a new shim similar to what you found out of some pop can metal but this time make it a bit wider so it reaches the ends of the saddle on the stem.

If the metal that is in there is thicker than a pop can shim then it means that he likely put a road bar stem on the bike. Road bars have a slightly bigger seating diameter than mountain bike bars. So if you need a shim that is thicker than a single layer of pop can metal then you really should replace the stem with the same length and angle but made to fit mountain bike bar sizes.