Current state of affairs:
Canon EOS R7
Canon EF100mm f/2.8 Macro
Canon tripod mount clamp
Sirui baseplate and lens support
Smallrig Arri style baseplate
Laminated bamboo boards and metal parts from a local DIY store
Negative Supply Basic Film Holder 120, Model II on a cardboard mask
Kaiser Plano, should be below the wooden base, needs more sweat though 
Note that the lens clamp is screwed directly to the Sirui baseplate. It improves rigidity and takes less adjusting. Boards are about 1 x 17 x 17 inches, brackets support loads of 150 pounds each, they were not perfectly right angled, so I had to adjust that with washers. They come in different thicknesses, or, if I wanted it to be more precise, I’d use blades from a feeler gauge as shims.
Total weight? About 20+ pounds? The wobbly part of the rig is still the clamped lens, but the lens support improves stability greatly. If I used a lens and bellows setup, I’d mount the bellow’s base to the vertical board without anything else. Any additional length of material adds elasticity and wobble.
The camera is in a cage which is not necessary with the lens fixed to the rig. It’s also turned 90 degrees to accommodate scanning 645 negatives with minimal pixel loss. For 135 film, I’d rotate the lens and would have to adjust things again. So I’ll do the MF material first and the 135 later.
For larger negatives, I’ll replace the R7 by a 5DIII and/or stitch several captures in post.