I'm thinking you may need to down sixe that fan. My incy is about the same size, pretty much the same design except I have side channels for the return hot air. I am using 2 computer fans rated at 8 cubic ft per min. All you want to do is just keep the air moving, stired up to keep even temps...
If I remember correctly, my LG turns from plus 30 to minus 30 degs. My homemade has false sides, very simulor to your's. Just finished my temp test and had less than 1/2 deg diff from any place on the egg tray. I have not checked top to bottom tray differiential yet. Will do that tomorrow. .....stan