Get yourself a toolbox. The most important thing is that the box won't let rain get inside, since the wiring can't get wet, both for safety and so it won't short out or rust your stuff. You may want to get a plastic box (like this one) that doesn't have too many rustable parts if you plan to leave it outside unattended, or use it in the rain. I have experimented with both metal and plastic boxes, and my current opinion is that plastic is the way to go.

My recommendation is that you go with a "dry box" which has an o-ring in the lid and is designed to keep out rain. They also have an extra lift-up compartment in the lid which is handy to put a switch inside. There are several companies that make dry boxes that are the proper size (longest internal dimension about 13"). This dark blue box is made by Flambeau and can often be found at Lowes or online for about $15. Walmart often carries dark green ones during hunting season.

Flambeau also makes these in other colors, which you can order online. Plano and MTM also make similar dry boxes for holding ammunition, which also have o-rings, an extra compartment in the lid, and are the right size. These also cost about $15, and could be good options. The MTM box has straight corners and a better reinforced bottom, but the latch and handle are not as nice as on the Flambeau boxes. The Plano box is available at Dick's Sporting Goods.

Have a look at this comparison of the top compartments of different dry boxes. All of these would be sufficient for installing switches inside, so it comes down to your preference.

Open up your box. Inside there will likely be a tray.

Take out the tray, and put it into a trash can.

Now we're down to business. All the important parts have to fit in here. Measure the interior dimensions of your box. This one measures 13" long by 6.5" wide by 8" deep. A 175 watt bulb with attached socket is 9" long, so your box interior must be longer than 9" at the very minimum, not counting mounting hardware, wiring, and foam padding for the bulb.

Buy a 175 watt Mercury Vapor (MV) light. This should cost about $25 at Lowes or Home Depot. Most of the major components will come from here. It's a pretty good deal since it comes with a bulb!

Open the MV light box. The package will include: a bulb, a lamp shade, metal transformer housing, photosensor, a bag of mounting hardware, and instructions. You can put the shade, mounting hardware, and instructions into the trash can, because we won't be needing them.

Have a look at the metal housing. This contains the porcelain socket to screw the bulb into, a plug for the photosensor, and the transformer. We're gonna take this thing apart.

On the top side of the casing is a plug for the photosensor. In order to take apart the casing, we need to unscrew the central screw in this plug (phillips head) and remove the plastic cover.

Remove the screw, take off the plastic cover, and set it aside for a moment.

Turn the casing back over, and remove the 4 large sheet-metal screws that hold the porcelain socket and the transformer to the metal housing. These take a 5/16" socket wrench.

You can now extract the transformer, the porcelain socket, the photosensor plug and the wiring. Recycle the empty metal casing.

Take a quick look at the circuit diagram printed on the transformer. Don't remove it. This may be useful to you if you mess something up.

Cut the wires connecting the transformer, photosensor plug, and light socket. Be sure to label both new ends of each wire as you cut it so you know where it used to attach!

A tag made of electrical tape and marked with a whiteout marker works nicely.

Now we need to attach the photosensor. Trace the photosensor on the top of the box, and mark the center of the circle with an X.

Using a 1 1/4" paddle bit, drill a hole for the photosensor.

Insert the lower part of the photosensor plug up through the hole, and then attach the top part to it with the screw. You may need to insert a piece of padding to get a tight fit.

The plug should now be installed.

Now, have a look under the upper lid. Note the position of the photosensor. Find a place in here where you would like to put a switch. The protection from rain makes this place an ideal location for a switch, and we won't need any waterproof switchplates or anything.

Trace a rectangular hole for your switch to stick through. You can either measure the size of the switch or trace the hole in a switchplate.

Cut the hole with a utility knife. If your box is metal, a dremmel works well.

Take the screws out off of your switch.

Insert the switch upside down and mark the holes for the screws.

You can are now ready to install the switch.

After you've screwed it in, it should look something like this.

Let's work on the bulb socket for a bit. Cut yourself a piece of wood at least as large as the diameter of the socket. This one is 2 1/4" x 2 1/2" x 1/2". Mark holes to screw on the socket to the wood, and pre-drill these. You should use a screw without a tapered head (like a sheet metal screw) since you DO NOT want to crack the porcelain!

Screw an eye hook onto the opposite side of the board.

After you've attached the socket to the board, it should look about like this.

Now, we only need one white wire coming from the bulb, so cut the other one off. I would suggest cutting the shorter one to give us more wire to work with.

Now we need to put in the transformer. Cut a 12 1/4" long section of a 1x6 or piece of plywood (5 3/4" wide) and cut of the corners. This should fit nicely in the bottom of the box.(you may have to make a couple of spacers if your box has a rounded bottom) Attach the transformer to the board with 3" #6 bolts. These are small enough to fit in the 4 holes in the corners of the transformer- 2 bolts will be good enough if you put them in opposite corners.

Sometimes, there will be glue clogging the holes in the transformer, so you may have to drill this out. Use a 9/64 drill bit. It may also help to countersink the heads of the bolts on the bottom side of the board so it sits flat.