I'm no expert, but I just converted a similar cooler to use as an incubator this next season.
First thing to do is get rid of the refrigerant. Ideally, this should be done by a professional to prevent injury, and to prevent allowing the refrigerant to escape into the atmosphere which is illegal. Unfortunately, mine fell off the back of my truck while unloading it, and the coolant escaped. Luckily, there was no other major damage.
Next, inspect the seal on the door and order a new one if needed. Mine needed it and the seal cost me $75.
Remove all the cooling system, and then clean it all up.
Measure the height of the inside, and round down to the nearest even numbered foot. Buy enough 11" flexwatt to run the length of the 2 sides. 11" flexwatt must be cut in 1 foot increments, which is the reason for the even numbered measurement. (I assume thats true for flexwatt brand, mine is Calorique).
You'll want a good proportional thermostat. I used Herpstat. Obviously, you don't need one with a night drop! If the cooler already has a flourescant light in it, you can use that as well. Also a small fan.
I used a fan that I already had, but I've heard a lot of people say that they use a 120v fan from Radio Shack.
I also used a power strip with a circuit breaker in it.
Solder or clip power cords on each of the two strips of flexwatt, and tape them to each side of the cooler.
Hooking it all up - The light, the fan, and the thermostat are all plugged into the power strip, and the flexwatt plugged into the thermostat. (The light and fan are not controlled by the thermostat.)
Run the probe for the thermostat down the back and lay it or tape it to the center shelf.
In testing mine, I've found that there is a temperature gradient of several degrees. Without the fan running, its hotter up top. With the fan running, its hotter at the bottom. I think if I put the fan on a rheostat, and slow it down, it should take care of that problem.
Mine has an advertising panel at the top that has space behind it. This is where I put all the wiring, thermostat, and switches for the fan and light. Doesn't look like yours has that, so you'll have to think about where to put that stuff.
Good luck!
Steve