I made 8 of these clamps about 30 years ago. They have had a fair bit of use, but now I intend to cut them down to make clamp heads, which can be attached to wooden bars. I had to struggle a bit to get them down from overhead racks.
These were made from library upstands, originally T-shaped. I cut off one piece, to make them L-shaped. The box section steel is too thin to sustain repeated use of threaded studding. Not having any welding equipment then, I inserted greased dowels through the holes drilled for 3/8” BSW studding, then poured in molten lead. This has actually stood up well, far better than the tommy bars. But have you ever attempted to tap lead? It’s not easy! I learned subsequently that I should have used tallow as a cutting agent.
The clamps’ feet were made from the brackets for the library shelves, which each had 3 prongs to insert into the slotted uprights, inserted into a saw cut in maple, then a pair of screws through holes in the steel.
Hope to post the clamp heads in due course