Found insulation foam, Celotex, Kingspan PUR type from Wickes, B&Q builders merchants , skips on new builds...... OK to use.

Cheap , comes in 50 and 100mm thickness natively, usually foil faces, that just peels off.

Its not enirely dimensionally stable, tends to cup like wood a bit when foil is peeled.

It does wear tools, at least wears hand saws used for cutting it in bulk, so will take the edge if new milling bits.

Still kinder than z being lower than expected ;-)

Dont think this is a general Poly URethane foam property, just insulation board may have glass strands or someat mixed in.

Can get PUR foams in all sorts of densities, its used as a core for surf boards and as model board made for pattern making and priced accordingly.

PVC foams are used in sign making but tend to be more like 5mm thick.