+1 for CPVC
The flow rate of your mash into your manifold can greatly effect your efficiency. Palmers book talks about it. I just looked at the pictures (big suprise) and made a CPVC manifold that is in the shape of a square with 5 4 rows of the pipe that i took a hack saw to and scored slices into in short integrals. Works like a champ, will not get crushed, easy to clean.