I have almost the exact same thing, except I actually use it for my cichlid tank. Works well. (Except I think I paid 12 bucks for it at PetSmart )
I dont understand how a square hole would be easier to make a hole? Got a drill with a 1/4" drill bit? Drill through fridge door, feed the probe, seal the hole with silicone, and glue or tape the display to the outside of the door.