Hi Scott,
I was inspired to do this from an image of an older Gary Birch bag a friend put me on to. I really liked the bag after looking at it a bit, but thought my flap attachment would be a "better mousetrap". Well, probably not. It's not my favorite bag design for the very reason of leaving two small spots for debris to enter. If you make the flap full width, it'll catch on the straps when loaded. It was fun to do as something different though.
Gary Birch's bag below.....much nicer than mine.
My strap attachment. Nothing you'd need a brain surgeon to sew. :-)
Top view of my bag...…
An image from Jim Webb's incredible book on bags and pouches. A similar idea, but not exactly.
I agree that a fat gusset bag with rear attachment would be a handful to manage. This one rides in nice and tight and balanced.
God Bless, Marc