Shell most likely would be easiest if you made it out of stiff foam, and then covered it with whatever looks like the turtle pattern. I'll assume looks similar to Bowser or something. So you would need shiny leather/plastic cut into the turtle pattern. Spikes are relatively easy, depending on how big they are. Even posterboard works, if you cut them out and roll them, but that's a pain. You could also buy stiff foam cones, but you'd have to find the right size, and then spray pain them and stuff.
Don't forget to carve into the shell, so it fits naturally on your back, as opposed to a flat surface that would have edges jutting out here and there.\
hope that helps. :?