Personally ...... I like your "H" inserts ...... I've made a few backhalf repairs with box tube .... I wittled a aluminum buck 6" long that I use as a fixture to make inside box tube inserts. It's basic but works & an acceptable fix. I actually cut a section ob box on the short angles opposite each other. Then clamp it to the aluminum buck. I remove the excess material then weld. It slips inside the box tube perfectly. I usually drill some rosette holes on both sides & presto ...... easy fix. I wish I had the pix of the last one I did.

REGS