I cut mine out of 3mm mild steel plate (300 grade). Depending on what thickness you need, you can buy steel shim material of varying thicknesses. The thin stuff (<1mm) can easily be cut with tin snips. Much thicker than that and you need a hacksaw or cutting disc. Better to make up the thickness out of a single piece, rather than stacking heaps of thinner shims together. If you google shim manufacturers, there are a few specialised companies around the country who can make shims to order. Most of them cut them with laser or water cutters.

Note I haven't tested my setup out yet, so can't confirm that it will work / there won't be too much flex etc. I think it will be OK, but would want to test before saying all's good.