I really don't think it's necessary for you to go out of your way to to bring the temp into the 50's for highlanders. You're probably giving it more temperature differential than most hobbyists...including myself. What species do you plan to put in there?

Is there a colder place you could put it and then just heat it during the day?

I doubt that this would do it enough, but a cool mist humidifier might do a tiny bit of help.

Also, placing the terrarium near a very low set air conditioner unit would do the trick.. just turn it on for a few hours at night and that would do the trick... In any case, as Petiolaris indicated, it will not kill the plants to be a few degrees off, so long as they have some temp differential night and day.