The shelf as it stands now.
Cutting the steel pieces
I cut the square tubing using angle grinder and cleaned off the mill scale from the ends, where it’s going to be welded, using flap disc.
Welding the pieces together
I held the pieces together while welding using magnets.
Welding square tubing.
Both sides welded.
Drilling holes for the rivnuts.
Inserting rivnuts with a special rivnut tool.
I decided to add one cross piece at the bottom of the shelf, to meke it more sturdy.