Shane Martin, a British artist, makes awesome sculptures using found metal objects. We've previously shown his cobra made from scrap metal. Most recently, he made this 3-foot long banjo with car engine valves, a bicycle gear, an old pair of pliers and strings from a bass guitar.
At best, this is a ukelele, but it would still need a fret board.
Without a fret board, you can only make 4 notes. one for each string. I guess you could use a Slide to play more notes, but that is not how a banjo (which this is not), or a Ukelele is played.