You just need a stopper on each of the side rails that hang over the edge of the table to keep the loom from sliding. That could be a pair of clamps or even the same type of L shaped rod that you already have going through a hole drilled in the side rail. A hanging shelf bracket could also serve as a kind of cleat by hooking over the bottom rail and hanging over the edge of the table. This could be made out of a few pieces of wood. Agree with the nonslip rug backer or shelf liner. You could also just put a board hanging over the edge of the table a clamp the loom to that. The clamp will act as a stop.