I figured you must have something like that hidden away in a corner that we had not seen before. I suppose you could use a jig saw but you would have to be pretty artistic to avoid squigly edges. Most other methods would leave a stressed edge. I'm surprised nobody else asked about it.
Now I have a question about the welding. Are you going to tac the joint and then remove it from the form to do the final weld or are you going to weld the whole thing while it is still in the form?. If the latter how are you going to prevent the gases from the burning wood contaminating the weld?