I have read through a couple of pages of the assembly so far.
A friend put me onto your site over the weekend.
Looking at the design and the bearing mounts if they are perpendicular at 90 degrees(Some look to possibly be 60 degree but I have to check again) could you not use square tubing for conduit? I think it would be more ridged along the run as long as it it not twisted for larger builds.
Pound for pound the round tube wins for rigidity but I have read people filling the conduit to reinforce it to prevent sag, which defeats the purpose of light weight.
Looking forward to building one myself