The dimensions of the key slot internally, leaves enough room to use the battery housing from the original Sony radio. Cut it out from the radio and carve down to size enough to hold the batteries in place and slide into the cylindrical housing. From then solder two wires from each battery output and wire the corresponding wires to the opposite sides of the 'plus' on the base. For my conductive metal I used pre soldered pads bought from Maplins (have posted the link below). Using wire cutters, I cut them to the size of the plus ends and glues them down solidly to the outside of the key base.