Hey Steven, I've played around with creating my own collectors in the past. When I was prototyping I actually used the galvanized steel studs you can buy in hardware store and sheet metal screw to attach the vertical and horizontal members together. I used aluminum sheet as a backing. Obviously, the big downside with galvanized steel is that it will begin to corrode pretty quickly, unless you have it nicely painted and kept that way. Also with the Al skin on the back I used a silicon gasket (caulking) to minimize the corrosion potential from two dissimilar metals touching. Extruded aluminum would optimal but it is, as you've discovered, hard to find a supplier and the right design.
You can then use cyana-I-don't-what-it's-called foam insulation board for the backing and the internal sides. The trick will definitely be to keep the glass in place - not too rigidly so it doesn't break with temperature variations causing everything to expand/contract. Again, I'd use that silicon as a gasket between the glass and whatever metal you decide to go with.
Just some ideas - I'm a total amateur when it comes to this but I thought I'd share some of my amateur-ish experiences. One thing I did gain from trying to do it myself was a complete respect for how hard it actually is for manufacturers to make a solid product - whereas I would be afraid to place any warranty on any of my creations.
