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Ortlieb Messenger is 30L, and that's big enough to be awkward. Pro is the same bag but with a transparent layer facing out for branding/maps, and all the internal accessories are included.
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North Face Duffel (XS = 33L, S=50L) could work, indestructible and comfortable enough for a short ride. By no means any use for longer rides, but they're great bags and I have several. Mountain Warehouse do a version that's half the price, but the shoulder straps aren't as comfy.
At 40L you're really looking at panniers or a full on hiking bag (these often interfere with your helmet as they come up quite high).
What's the best large (40 litre plus) backpack to get for short distance riding?
I've got a a load of gym kit to carry about and am struggling to fit it in my chrome messenger.
Ortlieb messenger pro?