The grow shop should have panda film, and IMHO it is superior to mylar for a normal-sized room because when mylar gets dirty, fugheddaboudit. Panda retains its reflective qualities far better, and is easier to work with because it isn't going to tear like mylar does.
The gap- eh, picture when you go into a grocery store, and you have to go through 2 sets of doors? It's like that. A light-lock. You don't HAVE to do it that way, but you can be SO much more laissez-faire about tacking up the panda when you have that mostly-dark buffer room in between to negate light leaks. Panda has white front and black back. It's GREAT for this purpose.
Hint: Get a staple gun...