Hi Vagelis,

Looks ok to me. . . . Just a Couple of simple things that will help.

You could drop those inner triangle braces and just keep the inside corner ones but make them little larger, there will be more room with others gone. Not much gain strength wise from them being there and not having them means less welding which will lessen the chance of heat distortion.! . . . . Would do the same with corner braces for the Bed frame just to help keep it square.

Also the outer End triangles will be better placed on the outside of the box section and made larger so there is more contact area, they will also cover the ends of the section. Then when you weld just put short welds but on both edges. Again this lessens the chance of heat distortion by allowing short weld runs and balancing the welds pushing/pulling action.