Thanks for the IGES to look at. Have you ever thought of doing an ultralight airplane tube frame like the T-Bird I & II?
http://www.indyaircraftltd.net/
I have a T-Bird I with a 28" narrow cabin. I need to widen it about 5" on each side for my big shoudlers and more comfort. :) They use OD 1.25" .065 4130 steel tube for the tube that goes over your head and the rest is 6061 OD 1.25" .065 Aluminium tube for the rest. The front half of your frame, maybe, less the side bars, or at least lower, with the front sloping windshield tubes moved out as far as you can, would look awsome on a T-Bird I or II. Be nice if they had connectors like PVC pipe has, made for Carbon Fiber tube and use Carbon Fiber pipe and epoxy glue to attach them together. Are you going to put a suspension on this frame? Make a neat off road go cart frame to, for us big boys, using a cheap Rotax 670 sled motor 106hp@7000rpm. :) If your interested, I have some pdf files of what the the T-bird airframe looks like. Armilite@aol.com
Rich