To keep costs down though I would avoid extrusion after all this is a play thing not a production machine. I bought this frame from Ukraine (Still not arrived! after almost 4 weeks)

http://www.ebay.co.uk/itm/2821250171...%3AMEBIDX%3AIT

you can make this from 1/4 sheet of ply and I intend to remake it on my cnc milling machine when I finish it but oversized so I can print bigger, also make it with 12mm not 6mm ply which means minor cad modifications.

I was thinking though of modifying the cad files so I can bracket every corner then it will be on par with the aluminium machines but a damn site better looking ;)

I have a thing for wood especially when varnished.