George, so you were using yours at the top? That's a good idea and will keep the bees from bridging the frames between supers.
Fordguy,
The main concern that stays in front of my mind is that beespace should be maintained. One way of doing this, that hasn't been discussed with me by anyone is to put slats in the shim what would be similar to a slatted rack. As long as the distance between these and the frames stay within beespace, your shim could be as think as you want it.... Sounds like a lot of work however.
Waya