Can't help with the mechanics of the build,Monk as I'm in the same boat as you at the moment,but here's a site for all and every piece of extrusion you might want for the build.
You have to completethe formto download two seperate PDF's one for Pricing and tother for profilesof what extrusion they do.

The drawback is...Minimium order is £100

http://www.valuframe.co.uk/