Ours has a screw down fitting to seal it but you only want to seal it when you are moving it. If you leave the tank sealed, it will suck in the sides of the tank until it eventually starves the engine
of fuel. If it gets hot it will bloat.
Once in place, you open the fitting so it is not longer sealed and can breath.
You shouldn't be putting the tank in an area that isn't properly ventilated.