I have the same mini fridge set up (3' x 11" flex watt & computer fan), minus the plexi front and the fan is at the top. It worked great! Only a .5 degree fluctuation. Used the regular Herpstat to run it.
I also think you should add a second piece of plexi on the inside and seal it well. That way you will have an air pocket to insulate it better and keep the temps as stable as possible. Nice to be able to see inside with out opening the door for those "I have to look, I have to look, I have to look!" kind of days.
A light is a great idea!I found my self removing the tub all the way to get a good look inside. It was dark.
All-in-all, looks great!!