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Just my tuppence worth...
I've got the Alpkit tapered drybag, which now appears sold out but I'd hope they get more in. It was £20.
It was pretty frustrating to begin with as if you had multiple items in it, the main strap round the saddle rails and under the bag would find the gap between two bits in the bag and work loose. To get it tight again you had to completely re-pack. Mega faff.
This has been solved by adding a piece of flexible plastic to the inside to line the bottom. I cut it out of half of an old A4 ring binder and shaped it a bit. Took 2 minutes. This stops the strap getting between anything in the bag.
I also added a second strap with a metal buckle that is really easy to tighten up. This means if it ever does work loose, it's 20 seconds to get it tighten again.
Cheers, probably a bit on the spendy side, given what I'm thinking of using it for.
I wonder if I could do something like JC's bike with a drybag strapped to the seatpost or into an adapted bottle cage? Lightweight, stores what I need, but no extra capacity and I don't think drybags are going to be as durable as say the cordura seat bag.