Haha, for that last comment I like you...hehe, and for your name.
Anyway i agree, if you got a piece of plastic sheeting 1/2" thick you could use a hack saw(or better) and a heat gun to make your block. Or use a thermoplastic and a mold.
Alternatively, if this is possible, use metal instead i would think you may be able to get a block for less than if made of PVC or do as your name says and fire up your foundry
Check dumpsters and ask for left over plastic sheeting, you may just be able to make it for free...
Good luck any way. My current design involves a block too, just is like 2 by 2"(its actual 40x40mm) and solid alli. I think i may have to re make parts of it though...
Ok happy spudding!