Looks like galling is taking place. (Probably two different alloys.) If there was some way to use through holes in the aluminum, the threading wouldn't be a problem, Two bolts can indeed be done with through holes and nuts. But I need to apply clamping force, and nutserts seem to require a tool for installation. There must be some simple way to do this. I considered making the threaded part from steel, but you can'get interior right angle steel channel. At least no one shows such; only radiused internal corners.
There seems to be some agreement that to do steel into aluminum requires a hole depth 2x diameter (which one of told me). So perhaps 1/16" bolt? But it won't take the clamping force, Or perhaps go to 1/2" wall that I am threading? Not sure if I can find 2.5" 1/2" wall channel.
3x1x14 ga. steel rectangle tube would work if I could slice off one edge. Don't see any practical way to do that with tools that I have or can afford for a prototype.
As usual, my wife solved the problem while snuggling with me: make my own arbitrary leg width channels. 2" wide steel plate on top, 3" wide plate on bottom probably thinner than the 1/8" aluminum I have been trying to use. A 1/2" thick, 1" wide steel plate between the edges of the other two plates; through holes on edge of top and bottom plates; tapped holes in side plate. Bolt it together. Bottom plate is now wide enough for the caster to bokt directly on; tap holes in bottom plate for clamping bolts which can now be steel pressing against an aluminum compression plate against the wood to be clamped. Less holes to drill; fewer matching holes required. Downside: drilling through holes and tapping steel; a bit more weight, but stiff.