Jim H - Man that would have made things easy! I wish I had the experience to cut stuff out like that but this is only my second model (yeah dumb I know).
Jim S - Dunno how you'd go getting a 90' angle using round tubes. 3 would give you 60' on each corner. Trouble is the epoxy would not actually penetrate the fibres of already cured tubes so ya really only got an epoxied joint with some stiff filler in it. I dunno whether you'd be improving the tensional strength of the joint much over an ordinary epoxy fillet. Just my two cents (which by the way is 2.0868
of your cents)