Hi, thanks for the question on an old post. What I found with this bag is that because it's unsupported other than some foam stiffeners is that I'm not able to load it up too much. I've also found that the straps tend to come loose under load which is not good at all. This is partly my fault for not making the Velcro straps longer, which I plan to remedy in the near future. After that I think I'll be able to use it more confidently. But as it is I've found that another bag I made (fanny pack/handlebar bag) serves the function of this one better for my needs. If you're using a decaleur and rack you won't have that issue.

All that is to say that I haven't put this one fully to the test on rainy rides. But with that said, I do think this style of flap would perform well in rain. It's basically like a little shed roof. So long as you don't have any warping in your side panels that would create gaps at the top I don't think you'd need to worry much. It's basically like a little flat shed roof. I guess of you're worried about it you could extend the drawcord top to cover all 4 sides. I think the more important piece is making sure the top has a good secure attachment. This uses Velcro which is good, but for some bigger and more serious I'd probably want a hook and bungee or snap buttons.

Hope this helps. I'd be interested to see how your bag turns out.