In reply to David Coley:
Back in the day, when aid climbing, just used two cows tails (with krabs) of strategic length, can't remember any issues with this system.
The old Whillans used to be better than a sit harness for comfort but more crucially reach, it had a lower cows tail attachment point.
This could be made even more effective for very long reaches by putting a short cows tail directly onto the sit strap between your legs. It meant you could reach even further than your vertical ape span.
Back on the adjustable cows tail, if the spare belay plate is a bit cumbersome , you could try making a "buckle" from two rings or chain links.