Ready Steady Eddy

[EddyCurrent]

instead solder 4 diagonals in such a way that the long and the short sides are separated each in 3 equal pieces, or somewhere near it

Solutions? Think in arcs. Like the door of a stone house or castle or so. 1 short horizontal bar each from one side, soldered 15cm below the rail supports directly to the previously suggested diagonals position will strengthen the arc, permit the diagonals to go lower at their low ends, and that combined with the removal of the middle vertical bar and replacing the bottom middle bar with diagonals will make everything stronger, resistant to twist and most importantly remove any push from the lower rectangular replacing it with pull, which in fact is way stronger. Or no horizontal bars , the rails supports are double and look quite strong in vertical direction...You can pull a lot but push very little :-)

silyavski, I knew you could not resist to comment on a frame design

I got the first part but I'm sorry I did not fully understand how to draw everything you said in the second part, but here is some of it