Glad to see that my idea has chances to work . I am a lot on the move and going from country to country with the job in Europe and don't have the luxury of having too many tools around, so whenever I need to build something I have t come up with weird ideas to make it work. Where I am now I have access to a 3D printer at the local library makerspace but there aren't any other power tools there like a drill press or saws. I would need access to a dry say to cut flush the square tubes so I can connect them end to end and obtain a square frame. Also a drill press would save a lot of effort when drilling the holes for the U-bolt, which are rather precise. Now I have to do all that with hand tools. What does 25kg of force mean for a vinyl cutter frame? Is that per u-bolt or per leg? There aren't any catilevered forces being applied to the frame, all the pressure from the locknuts will be exerted near the 90 degree bend of the square, so that area can withstand much higher forces I would estimate. I guess that when all the members of the frame will be fastened in place there will be no movement in any node and that it will not be necessary to tighten to extreme these M4/M5/M6 ...DIY bolts.