Patching waterproof panniers I've managed to tear a couple holes in my panniers. Nothing enormous; they can still hold their contents, but they definitely need patching.

What would be a good way to patch them, keeping them waterproof and hopefully durable?

They're Detours, 50% nylon and 50% polyester, just simple one-piece bags. They're several years old, and they were a gift - haven't managed to find the exact same thing online.
The reality is that panniers are very similar to camping equipment. You have, "50% nylon and 50% polyester". So go to a camping store or online retailer and find a patch kit designed for that fabric combination. It may be included with the kit, but if not, you'll need a sealing compound to cover the patch.