I was thinking that it would be easy to mill .5" inch aluminum sheets into the plates. Thinner would be better to start, really you would be cutting the path into the sheet, then stacking the plates, rotating each one 180 degrees so that it goes wort, water, wort, water. I think McMaster Carr sells the high temp silicone as sheets as well and that could be cut to fix the outside seal between each layer. The only important seal on the inside would be separating the wort and water, as long as the water and wort path are restrictive enough there is no need to have a perfect seal on the inside. I will try to design an example later tonight.
As far as bolting it together, the first and last layers are over sized with maybe a steel reinforcement layer and bolts on the corners to pull it all right together.
Be nice, first time using sketchup and in bed, with my mouse on a book.
http://sketchup.google.com/3dwarehouse/details?mid=235a009eaaa2956078060da0cc5e23a8&prevstart=0
same Idea as the above picture, just reverse each plate every tier, top and bottom are the ones that need threads, shouldnt be to hard, flat silocone with holes punched in all four locations should work fine for a seal, and clamp it down with bolts and a steel plate at top and bottom