well, unlike beer gasoline is explosive, and it's really hard to seal the tank so that it doesn't leak without one, but I guess you could run without it.
By using the parts from a fuel cell, you get a bladder, fuel door, and the right venting.
I'm still working on mine, but the one we did for a friend of mine wouldn't work till we vented it with a vent cap....then it looked like crap, and the one from the cell is flat.