I ended up not liking my work on the previous seat mount setup, so I started over today. I took a fresh piece of 2" flat steel, and started measuring and marking it to get bolted both to the frame of the car and to the seat bracket.
We're getting a little closer here. All the green pieces lying around are marking tools I 3D printed to help me get lines drawn on this piece of steel. Here we see the first two bolts in and all of the holes drilled.
And here it is all bolted up. It's not quite done yet - it's going to need a bit more work there on the aft end to trim it to length and get a tab welded on to mount the harness to. I'm designing as I go. But the holes are drilled in the frame, so this is where it goes, forever.