Dunno if these are big issues but off the top of my head...
You will lose the insulating features of the chest freezer by removing the top and building your terrarium over it. Dunno if that is a problem for the unit running longer to keep the set temperature. Would increase operational expenses at least compared to putting a top on and using in that manner.
Condensation may be a problem on the outside of your terrarium? Guess that would depend how cold you have it set inside and the humidity on the outside..
Interesting idea though.