
Im toying with the idea of just getting a car chair and plonking it on my frame of my old chair
That's not a bad idea actually.

For that kind of price, a secondhand Steelcase Leap or Herman Miller Mirra is surely a better option. They are designed for extended periods of sitting.
It might not look racy, but it's going to be better for your body in the long run.
The hard bit is picking one. Trying to find a sensible seat.
Im hoping its going to be fairly easy to attach, bit of wood on the bottom bolted to frame and attach it to the subframe of the seat. Just need to make sure it balances