First hit in Google on “Muscle box“: http://www.sailboats.co.uk/Cat~RWO_Standard_Muscle_Box_4807.html
Though a muscle box is a neat solution it does have a lot of friction because it has eight (and sometimes ten) sheaves. I use a muscle box but I have to pump the tension on with two hands. Today most W-sailors prefer a cascade system on top of the CB case. Because it has only three blocks (8:1 purchase) it has far less friction. The cascade is usually placed left and right of the CB slot leaving the slot itself free. A piece of bungee holds the cascade in place when it is not in use, without it the lines quickly become entangled. The last block of the cascade has a hook to attach the halyard or the storage bungee to.
A cascade system for a boom vang is explained here: http://www.wayfarer-international.org/WIT/race.related/RiggingTips/TonJaspers/TJcascade_vang.html It shouldn’t be difficult to imagine a similar system on top of the CB case to tension the Genoa halyard.