I'll make this one compact, just 12" long, same as the rack top. Will tweak the end shapes though based on this tester. Should be enough to stuff a light jacket and some food. For overnight trips I've got that weird shaped black bag in the pics above.
you could probably go a little taller still (and stiffen the whole thing a little with foam) and maybe hide the zipper a little for some resemblance of waterproofing?
You could probably elongate the front section up until the seat post. Or was the plan to stay compact?