You can already get these things (Sans jet propulsion) on the open market. You don't hang below it though, you're strapped in quite tight.

At the parachute centre run by a relative, someone decided to try one out. It was only when they started to calculate the forward/down ratios that they realised that the guy was going to have to register an actual flight route with ATC - he was going to fly outside of the dropzone boundaries within about four minutes!