Thanks. That's actually quite clever. I've seen full octagonal roll-over frames used on Porsches, but never anything so minimal. I guess one issue with it is that it's much more limited in the range of access options e.g. the sill is either on the floor or near head height and if you wanted to get to the R/H side sill, you'd need to take both frames off (assuming there's another at the rear end not visible in the image) and reattach the other way round. But if you just need something simple to provide access to the underside, it's neat.
I did actually contemplate building something out of scaffolding tube and connectors. It's relatively inexpensive, doesn't need welding so can be reused for something else when you're done and could also be designed to be modular i.e. reconfigured as a dolly etc. However someone then kindly pointed me in the direction of mk2mania who offer quite a versatile, well thought-out looking jig for £340, which seems reasonable, so I'll probably go that route.