I bodged one up from a standard LED stop/tail replacement lamp bulb. Wired both 'stop' and 'tail' LEDs together then took a feed from the auxilliary socket to a switch on the bars and back to the bulb. The individual LEDs have a narrow beam angle and so there is no need for an additional lens or reflector. Soldered the wires to the bulb and then encased the whole lot in some heat-shrinkable sleeving with some silicone to keep the water out. The entire thing is only about 30mm in diameter and maybe 50mm long so can be mounted anywhere you fancy, just need to align it properly so it's pointing straight back down the road. It's much brighter than the standard stop light on the bike and seems to fend off blind car drivers when it's grim on the road.