How are you planning to attach the steel plate to the tubing?
The frames on my U1’s are circa 1994 and at that time the frames had a metal flange welded to each tubing. The panel was then attached to each flange via screws along the entire length of the flange. The flanges also have a 90-degree bend at the bottom end for the panel to sit on, same as your doing. From what Roger has told me they did away with the welded flange do to difficulties the welder had in positioning the flange to match the shape of the panel. Soundlab now uses a plastic flange which makes it easier to properly mount the panel.
Also, my frames originally had no upper cross member which allowed the frame to flex. Soundlab added a welded crossmember in later production which greatly stiffened the entire affair. Attached are a few pix. of the frame.
Good luck with your build, you too ernestov!