Hi Nico - Thin square tubes are very stiff globally but not locally. You are better off having a very thick angle in this position then a thin tube. This area is shear dominant and you need thickness to stop this area deflecting. If you do want to use those tubes then they need capping so they do not lozenge... but I'd go a thick angle, thickest I could get.
Similiar theme - the pads that the rails are on need to go out to the sides. The little bit in between the radius and the pad deflects quite a bit relatively when you analyse it. Now is your chance to stiffen it.... Cheers Peter