There are plenty of plastic types that work well for CNC machining. I've had good luck with polycarbonate and acetyl; both cut cleanly without building up on the cutter. Is there a specific reason you need PCTG? I did some searching, and it doesn't seem to come in 2" thickness; can you laminate thinner sheets to form the blocks you need? It's also available in pellet form; I suppose you could have some blocks injection-molded, or injection-mold some near-net shapes and just finish them on the CNC.