I was thinking of something along the same lines, but all the roll-top drybags I’ve owned aren’t airtight – they keep water out, but if you sit on them they slowly deflate – so I was thinking of a closure more like the slide-on clips found on hydration bags (Camelback, etc.). I don’t know how to make one of those though, and it would have to be quite long to be useful (at least 50 cm), so I put the idea aside. Maybe someone else can think of a better solution though.
Cheers!