This is the Rambone slingshot, designed by the crazy german, Jörg Sprave. He shared a STL file of the model, but it has holes in it and some other inperfections. Also its very high res. I cleaned up the holes, optimized the model a bit and made it a true solid.
You can get the original file here:
Heres how to attach the elastic band: http://www.simple-shot.com/pages/tutorials-diy