I just finished making this wire spool holder. I designed it in two versions: one that clamps onto the end of a narrow shelf, and another that screws directly into the wood (either on top or underneath).

The main rod is 15mm and 23cm long it's threaded on one end with a cap on the other to keep it from sliding out. 

I also made sure the Y-frames were designed to be pretty beefy/robust so they won't snap under a full load of wire.