I'd pay careful attention to the likely diameter of the drybag once filled. IME, larger drybags can end up rather unwieldy in cross section. Pretty sure you'll be able to find something suitable in the 12-14l range.
@Bearlegged started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
I'd pay careful attention to the likely diameter of the drybag once filled. IME, larger drybags can end up rather unwieldy in cross section. Pretty sure you'll be able to find something suitable in the 12-14l range.