It was dead easy to make. All you need is the playground swing frame welded to the ground. I welded two concrete pipes together, positioned them at an angle inside the swing frame and welded a dumpster lid to the base of the two pipes. I used plenty of dynamite (welded into place) at the bottom of the 'barrel'. I originally intended to have the elastic ropes as a recoil mechansim, i.e. the barrel would be hanging directly from them, but as it turned out I didnt bother.
My next canon will probably feature this, but I ultimately want to design a ripsaw shooter!
I will give you the Saved Game shortly after I do all this college crap.