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I've been looking for a bag since my trusty northface holds more water than it keeps out.
I stumbled across their site and reviews seemed to be highly positive even though they are cheap as you like. I'd opt for a 20l because of the compression straps as I don't always have a full bag.
EDIT: Due in stock mid julyThe lomo seems the second alternative in the price range but the zip compartment although ideal isn't 100% waterproof apparently.
Not sure if this has already been mentioned, but Alpkit do a nice range of bike packing bags that are ethically sourced and will not break the bank:
https://www.alpkit.com/bike-luggage
I have one of their "Fixie" backpacks. Naff name aside, I use it for my daily commute (30 mins on folder - 1 hour on train - 10 mins on folder each way), and it's ample enough to carry spare clothes, laptop, etc and has lots of handy things like a zipped waistband for carrying keys etc:
https://www.alpkit.com/products/fixe-backpack
Just my tuppence worth, I have no affiliation to Alpkit beyond liking their gear.