get some corrugated cardboard boxes (bigger the better) and cut them across the corrugations about 4 - 5 inches thick. get 2 bits of wood about the size you want the target and drill 2 holes the same place on each of them. thread both holes on one of the pieces with threaded bar and secure with a nut. then stack the cardboard strips onto the wood, piercing them with the threaded bar until you've got the right size (or you run out). put the other piece of wood on top, then stick a couple of nuts on and tighten as far as possible, so you end up with a kind of sandwich effect. cheap and last for ages too.
hope this was a help
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