Ah yes, I did mean adding an extra bend. But I can see how that would complicate things. I’ve bought a cheap PlanetX seat pack for now, as framebags are already a bit daunting. But might see about modifying it to take a similar harness for starters.
Sounds like a chunky molded zipper with a flap over it would be a good option? I don’t think I’d need absolute waterproofness anyway, the stuff that needs to stay absolutely dry can go in the dry bags on my bars or under my seat.