that looks perfect man! i too was thinking about some kinda float to attach the captured/killed fish to for the same reason lol. it sucks to have to go back in each time you get something. from the looks of that bag, it would seem that you can even find a metal ring with some weight to it that fits the circle at the bottom to keep it completely open (like a cylinder)..
ive thought about using an old bodyboard and cutting a nice sized square of it.. you can drill and insert the dive flag, and even drill and attach the mesh bag if you want. a cheaper solution, but not sure how well it would work out. im sure it would be fine.