I have a growroom in a attic and I have the flowering and vegging rooms blocked off with black plastic. Black painters plastic is cheap, and if you seal all the edges with tape it is completely light proof.
In your case you could use two pieces of plastic and leave a flap in the middle so you could get in or out.

I line the inside of the black plastic and all of the walls with mylar, so i get better light reflection.

You need to make sure that the rooms are completely light sealed, or else you can run into problems.