I think something with the corners at the bottom sewn for some volume and then slightly wider at the top. Maybe 11 inches wide at the bottom and 14 at the top. Drawstring top. Reflective loops at the bottom, mid, and too sides are nice to help hang or tie them on a pack as well.
I have tag bomb bags and like them very well but they way they sew the top drawstring it makes the opening smaller than the rest of the bag.
I was thinking this exact thing last night. I'm sending in the dimensions to the manufacturer this week for this year's test bags, I'll try a couple like this.