You could cut out the bottom, and maybe some of the side up a bit to get rid of the thickest part of that bottom corner. Then cut a "wedge" out of the side to allow you to pull the sides together into a tighter (if lopsided) cone. Drill and bolt as you pull it together so you aren't fighting to get a bunch of predrilled holes lined up.
If something closer to the size of the one the OP made is what you want, you could use the barrel side to cut out a more traditional cone pattern.
As drawn, that could be made as follows: At a point where the straight side meets the bottom curve, drill a small hole and put in a nail. On a piece of string, mark 2-1/2" and 22", and pivot on the string to transfer those onto the barrel. Measure down an inch on both sides of the semicircles and connect them with straight lines, this would give you enough for a 2" overlap. Cut out, leave in the sun to get warm and bendy, then start pulling it together, drilling and bolting. I would likely start at the big end.
For that matter, if you had access to an old aluminum road sign or two, you could cut out the same pattern and drill and bolt it the same way after making a series of bends over a sawhorse or some such...some towns give their old signs away, others guard them jealously, so it would depend on what you could find.