Ya, making a little sound proof box is your best bet. Make your box but dont put on the top yet. Take another piece of wood the same size as your ends and cut out a slot on the top big enough to fit your cables through. Nail it in the middle of your box as a divider. Cover all the inside with some sort of sound dampening material. Put a hole for your cables in the bottom on one side of the divider. Place it on a outside, bottom corner of your booth and put another hole on the other side of the divider and through a bottom corner of your booth (It would probally be easier to not add the divider untill this point). Nail the box to the booth. Feed your wires up the hole, over the divider (cables in the little slot)and through the hole into your booth. Nail the cover on the box once all your cables are through. Having the divider and all the sound dampening material will keep it more soundproof.