Tall guy –
It sounds like your keg box is insulated similar to mine, and I even have a flat-back fridge. I wouldn’t bother with adding new insulation, but it might help if you purchase a temperature regulator connected to the fridge power supply. You can buy them on http://www.beveragefactory.com. I don’t have the digital, its just a dial that I set to 40 degrees (I have Guinness, so it’s a little warmer). You plug the fridge into that, and then plug the regulator into the wall. The copper mercury bulb drops into the keg-box (just insulate around it), and when the temp hits 40, it will kill the power on the fridge. It kicks on when the temp rises above 40 again. I have the power supply/fans continuously running to circulate air.
My only problem is that I have to empty the drip tray once/week. Not sure why i still get drips from the freezer plate, I have the fan system set up similar to yours. It still works fine, the compressor does not get too warm, and the temp stays right about 39.
Hope this helps. Ron