As others have said, condensation is caused by ambient air entering the fridge. Either by continued opening/closing of the door, or by an air leak. In a standard fridge there's a fan and a drain / evaporator system for the condensation, but in a converted chest freezer there is not.
Make sure there are no air leaks, and then stop opening the door! With a new system I know it's tempting to "check under the hood" all the time, but the less air you exchange, the better. Put something heavy on top of it so you aren't tempted to open the door.
Make sure all the seams are caulked around the collar, the weatherstripping/gasket is in good shape, and any wires/tubes that come into the system are sealed up as well. Just grab a tube of caulk and go to town.
Once it's sealed up, make sure you have a small fan inside, and at that point a small desicant system like the Eva-Dry should be able to take care of it. I have mine right below my fan and it works fine, I know others have attached the fan directly to it which should make it work even better.