I do not think you have enough heat tape in there. It should go all the way up the back wall. And you do not want to have any of the egg boxes sitting directly on the heat tape so you'll need to fashion some type of shelf extension on that hump.
The fan should be on the ceiling just in front of the heat tape and blowing down. Don't skimp by using an old computer fan. Spend the money for a new 12v DC fan from Radio Shack. You don't want the fan to fail after the eggs are in.
You might have more even temperatures from top to bottom if you move the flewatt up to the flat wall in the back and place a fan blowing air along its length.
You have to test it with the fan and with water bottles and egg boxes inside. The bottom boxes may get hotter than the top. You want it to be pretty even. Within one or two degrees.