Step 1: Bill of Materials
1. MR11 Sealed Halogen bulb
I suggest a 12V 15W spot for a good all-around light that will illuminate your path well. If you are more concerned about lifetime, you can get by with a 12V 10W spot. Stay away from the floods if possible.
12V 15W Sealed Spot
12V 10W Sealed Spot
2. PVC Body Parts
I have not had any trouble with the regular white PVC failing or melting at up to 20W. Over volting a 20W from 12V to 15V for more light output may cause softening of the PVC since you are effectively upping the wattage from 20 to 31W.
Sources: You can get these parts at any home store such as Home Depot, Lowes, Menards, Do-it-Best, etc. Look for 1" PVC couplings and misc parts. You are looking for 1" threaded FIP cap and 1" threaded male adapter. See pic below for an idea of what you will need.
3. Bulb socket.
You can probably just solder on wires to the bulb but the correct sockets are available from batteryspace.com for around $1USD so why not just get it.
Source: Ceramic MR11/MR16 Bulb Socket
4. Trailer connector
It is best to have a simple way to disconnect your light from the wiring harness of your bike which includes the switches and battery. I choose to use 2-wire automotive trailer connectors which are designed to handle large currents and operate in wet and dirty conditions. See pic below to get an idea.
Source: 2-wire trailer connector
5. Metal mount.
You can use whatever you have handy to make the mount. Since I wanted a light and simple setup, I used a single piece of light stainless sheet I had laying around. Bent it into a clamp and added a bolt through the rear PVC cap as pictured. A bit of rubber cut from an old innertube will protect your paint from the clamp.
Source: Whatever you have around.
6. Waterproof switch.
I have the switch permanently mounted to my bike between the trailer connector to my light, and another trailer connector to my battery. This allows removal of the light and battery as needed and the switch and wires can be cable tied, duct taped, or otherwise mounted.
Source: Waterproof switch
Of course you will also need a suitable battery. These halogen bulbs are rated for 12V but you can run them at slightly higher voltage to get a lot more light at the expense of reduced bulb life. Many people run halogens at 13.2 or 14.4 volts depending on their choice of battery, NiMH or Li-Ion. The building of battery packs is a detailed topic in itself and can be found discussed in great detail elsewhere. For battery cells or pre-built packs, www.batteryspace.com is a great starting point.