It does not need to be a rectangular block. The manufactured version is that way as it is cheap to mass produce using a casting.
You could make four cylindrical risers, one for each corner, maybe 2" diameter each, with clearance hole for the securing bolts. They could be made from 50mm PVC pipe filled with epoxy concrete. They are in compression, so the epoxy would work. Any height discrepancies after casting could be sanded away.
If you bought a piece of 2" or 50mm square cold rolled steel, cut four cubes out of it and drilled, the manufacturing tolerances of the cold rolled over an 8" length of bar would mean the height difference between each part would be negligible.
You can buy a whole lot of shims for not much money, all grouped together and labelled. They are called feeler gauges.