PE flanges are a good option, and I'm a huge fan of the aluminum 'armature wire' from my local Michaels store.
I used aluminum tubing to make collectors on these headers, but they only had 2 pipes per side:
I'm keen on trying the heatshrink tubing collector idea. I figure you could use these 'bead cones' as collectors as well, by lightly crimping the big end around the pipes going into it, trimming them to length and attaching PE flanges: