[Wavery]

1/4" x 1 1/2" Steel flat bar is pretty easy to find at scrap metal yards. Drilling it on a drill press is pretty straight forward too.

I would (heat and) bend it to a 90* angle "L" so that the longest part extends from the frame to just beyond the outer box wall. The short side of the "L" would be about 3" long and bolt to the frame with 5/16" self drilling/tapping bolts. I would weld a gusset to close the "L" for maximum support. The long side of the "L" would through bolt through the box floor frame with "T" nuts on the inside of the box (inside the cabinet). Might even have to weld some ears onto the "L" bracket so that 3 bolts could go up through the floor (about 1.5" apart).

S/S is soft and bends bends easily. If using S/S one would need to increase the bar thickness by about 50%. It would be prettier but not necessary unless one plans to use it in sea water.