I would not use ball valves, I would cut a hole in it a little smaller than the size of a tuna fish can top or whatever and then attach the can top to the drum with a single screw so you can adjust the flow, kind of like a shutter.
Maybe not use a tuna can lid, but you get the idea
Maybe some kind of sliding door arrangement
tell ya what though
for a totally new untested design you did damn well on your first cook!!!