I have a cooler incubator that is the same height but not as wide with just a single glass door. I bought my Flexwatt heat tape from Reptile Basics, they cut it to size and assembled the cords before they shipped. I initially put one strip down the center back and it wasn't enough. So I put one more on each side so tubs are surrounded on each of three sides. Now the problem is the front glass door, about one to two inches away from the glass door the heat in there is about 86F, but all other places in the back it's 90F. It makes me think I should have run heat tape even on the front door. And if that's the case I probably shouldn't have gone with a glass door fridge, probably should have went with a regular refrigerator with more insulation. I have two pretty good size fans in there too and it's OK, I just have to be sure to slide the egg boxes back away from the front glass door.
Also, you should be able to control all of the heat tape with just a single heat controller, I use a VE-300. Doesn't really matter if there are differences in size, just put the probe on one of the biggest heat strips for better control.
You'll have to be sure to set it up and let it run for a few days to equilibrate, then take a temperature probe and place it in a different spot every day and 'map' the whole incubator to see if there are any cold spots. It took me a few weeks to get everything under control.