If you really want to get fancy.... look at the picture / diagram of the 1 piece swirl & expansion tank in the upper right - in (Photos and Video (Clips)).
Looks like a cool design... my only question is how do you figure the volume required for the expansion tank?
Anyway - there is a drawing and a picture there. If you are going to have something fabricated, maybe you could try it, and give us a report!
Rocky