couple 4th of july's ago my 2 uncles had a couple of those mortors. They pretty much took like air conditioning duct pipe, duct taped the crap out of it, and jammed it in the ground.
They dropped one mortor in the pipe and light it. Needless to say the pipe only worked once sending the mortor about 20 feet off the ground and showering us in sparks. It was the funniest thing I've ever seen in my whole life.
When we checked the pipe it had a giant gash down the side of it. I'm surprized it actually shot up that far into the air haha.