Making a DIY Courier / Messenger bag

Ah I think I see what you mean - looks like the corners are darted so that it curves around the bag...
Don't think that would work too well with compression straps etc - as they change the depth of the bag. Also with suspension strapping on the top - this should pull the flap round the corner avoiding it, so shouldnt be neccessary.

Might be worth picking up some seam sealant (tent shops sell it) to use on your bags seams - should help prevent water getting through (although it will wear off eventually)

The suspension strapping does pull the flap a bit around the top corner, but nowhere as much as for the bottom corner in all cases I have seen (i.e. my bag and other people's bags I had a look at while waiting at lights). In fact I think project tortoise bags had the webbing attached to the flap on the top corner to pull the flap closer to the bag. What I can say is, on my bag, which has suspension strapping, I wish the flap got much closer to the bag when I tighten the straps.

Ta for the sealant suggestion... I'll probably have a go, though water ingress happened only once in 2 years, and under the very very worst rain I had to go through (I know I'm retentive, but I like my stuff to stay dry in all conditions ;)).