Apart from Pete's observations about the seat, the only thing I would say is to turn the edges of the flat steel over, or at least into a curve. If you come off it's going to impale you!!!
But there is nothing wrong with this at all as a concept, and with one or two minor modifications, it's a great solution.
Mod's come in all shapes and sizes, some good, some bad - I like this, great way to recycle, and it gives you a nice cheap solution to a problem.
