Where would the poo go? The only thing I can think of is that it would back up until it comes out your mouth. (Although I can think of a few people that talk shit, maybe that’s what happened).

I don’t think welding would work. You’d have to stitch it shut and wait for the flesh to grow together, which might take a few weeks. I don’t know if cauterisation would count as welding, and I don’t know if the seam would be strong enough to resist the back pressure. If it tears open due to said pressure, there’s also the risk of infection due to foreign matter entering the bloodstream.

Why don’t you start with a cork or a butt plug and see how well it works after the first twelve hours? Then if you like the result, apply superglue for a quick fix.