Update 2018-07-09: tube_end.step uploaded for those who needs to modify for your tube
This is a redesign of : https://www.thingiverse.com/thing:2681839
Instead of carbon fiber tubes, this uses 1/2 inch aluminum tubes that are readily available in most hardware shops. Beefier brackets.
Parts printed without support at 0.16, 100% infill using PLA
2x 1/2 inch aluminum tube 58cm
6x m5 16mm
4x m5 T nut
2x m5 30mm
2x m5 nut, locking type recommended
epoxy : http://www.lepage.ca/en/lepage-products/epoxy-glue/marine-epoxy.html
spray paint, your choice of color
self etching primer : https://www.rustoleum.com/product-catalog/consumer-brands/auto/primers/self-etching-primer/
1) at one end of the tubes, glue tube_end about 35mm in, wait until glue sets
2) install brackets, tap frame m5 holes
3) once the glue has set on one end, glue tube_end to the other end and install to the brackets and frame, let glue set. Don't use the printer while the glue sets.
4) optional : remove, prime aluminum and paint, re-install. Or you can paint before glueing
for 3 above you can do this for the 2 sides at the same time OR to make sure that the lengths are the same do it for one side first then when the glue has set, use the assembled piece as reference for the other side.