Just cut a length of 1"X 3/16"-1/4" aluminum bar stock that fits between the gears. 2.5" maybe? Mines at the shop, I'll try to measure it when I'm back. Don't use steel, you want it softer than the gears.
If you don't have any... try an auto, machine, truck, welding, metal shop. They'll have it in their scrap bin.