You can use 1x2's from your local lumber store. I cut an old med box into three shims.
I like to drill a few 1/4" holes at an angle in the shim for some added ventilation. If the bees think you overdid it, the'll just fill them with propolis.
Lay a peice of newspaper on the frames and place the shim.
Pour about four or five pounds of sugar right onto the newspaper and spray lightly with some water.
This is a little different shim with a wire bottom.
Since I popped the tops on the hives after the bees had sealed them up I repaired my damage with a little latex caulk.