We had a kid who was trying to get their Huffy Motorcycle converted to single speed because I think they can't shift the freewheel or don't know how or their derailleur is broken because I don't know maybe the Huffy wasn't designed as a foo fightin' lawnmower and is barely functional as a normal bicycle.

If I was doing a conversion to single speed I would want to make it look really nice and perform well as a shop. I wouldn't want to do some home-brew stuff as I normally would consider for something like this. If I am looking to go cheap I would do it at home, if I want it to be super awesome I would give the shop a reasonable budget and let them build something cool.