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Not sure. I've read that anything sewn can never be 100% waterproof, because by nature youre putting a 'shitton' of holes into it (technical term). If you don't have much experience with sewing I wouldnt worry about waterproofing to start. I used a waterproof zip and waterproof fabric, but the seams are so badly done that it would fill with water pretty quickly. Getting even seams in corners is much more difficult than I anticipated
TLDR: maybe
Inspired by some of the DIY in this thread I tried my hand at making a little top tube bag. £5 worth of cordura, £1.10 for a zip and £1 of velcro. Its quite uneven and a bit asymetrical, but works fine really (except it wont be waterproof whatsoever). Happy with it for a first attempt: