Remember, Use your own common since, and build this ONLY if you know what you are doing

 

Get a steel tube approximately 4" ID X 18" Long X 1/4" or 3/8"
Get a peace of flat steel the same thickness cut to the size of the tube
Weld the two pieces together and drill a hole at the very bottom for the fuse
Weld on two pieces of Rebar for legs at the desired angle
Get 4 pieces of 1" X 4" Foam and 1 piece of 1/2" X 4" Plywood
Assemble the wad with 2 pieces of foam on top and 2 on bottom, cover fully with Duct Tape
Load Cannon
Light the fuse and watch the fun