the cheapo phissionkorps way:
get one of those cool mist humidifiers, cram some surgical tubing into the business end of it, put some waterproof tape around it to seal...snake the tube up into your terrarium (the straighter the better), tape the tube to the frame or something, and there you go.
The expensive(r) way:
all you really have to do is divert the flow of the coolant. Instead of through the fridge, it should go around your grow chamber, which will require bending some same diameter copper tubing around the grow area, then sautering the two ends of that to the two ends of the copper tubing in the back of the mini fridge. I know thtats not the best explanation, btu thats more or less it