filkry

This looks like an ideal option to try, and quite affordable. People in the Amazon reviews say it comes with everything you need too, which is great. I think I'll try it.

The tubus mount and adapter set also seem like very good options, but this one seems like a good mix of all-in-one and cheap. I'd feel more comfortable putting a pannier on this then trying to fit into a trunk back on a seat-mounted rack.

Thanks so much for everyone taking the time to offer advice. I feel quite welcome here!