Thank you. Good point about the reverse extension; I doubt it will be a major problem, but it will be interesting to find out.
My understanding is that a single plate would cost a lot, certainly by comparison with the bass it's meant for.
I'm also considering a hand-made involving a steel bar, a drill, a countersink, and a couple of longer screws, plus a spray can of Hammerite. It would be fun to do, I reckon.