I say hook an ac unit up to the end of it, and use it as a ferm chamber. If it is only a couple inches shy, you could cut holes in the top and let the top of carboys/ airlocks just poke through the box. If you insulate it/seal up the holes well enough you may not need to make that collar.
Edit: I would first make sure you could fit the 4 carboys in there. If the space ends up being inefficient for your equipment, just use it as storage or a beer chest.