I finally got seat belts installed on the rear luggage tray. I didn't take enough photos for a how-to, but here are a few to help the next guy.
I had some very experienced local help who restores old VWs, BMWs and Porsches.
The bolts are spaced around the frame and tranny for easier access underneath. Steal spotted to the frame and bolted through the glass. Not bomb-proof, but good enough for local drives.
Disclaimer: I know my place in the automotive food chain. These seats are only used on neighborhood roads and my youngest ones don't get much further than a couple blocks until they get older. I learned it's pretty much illegal to transport kids like this unless they're 40lbs (or in an emergency).