Go to a hardware store and find some light gauge aluminum tubing that just fits the bulb diameter. Cut a (~)1 1/2 inch piece. Crimp the front edges of the pipe to keep the front of the bulb from falling thru. Cut several 1/4" to 1/3" slits in the back end. Attach a connector and its wires to the bulb terminals. Drop the bulb into the pipe. Fold the slit pieces of tubing in to hold the bulb in place. Attaching the assembly to the handlebar is left as an exercise for the builder. (caution: it might melt the helmet) Add a suitable connector to the wires and put it in. Ride.

Better. There was an ad here last night for a company selling $49 and $59 lamp holders for that type of bulb or maybe MR16s.