[FWB]

mine has some travel, but the grommet is sealing. if yours isn't there may be something up with the spring. the screw in the middle just removes the disc and there is a spring floating in there keeping pressure on the disc, yours might be broken or rusted to the point of it not being....well springy. the springs aren't anything special, it just has to be strong enough to keep enough pressure to seal and it works in conjunction to keep the latch closed.
if you look inside the filler neck you'll see it is a few inches long, i was going to drill and tap from the outside-in, just low enough to put the hole inside the trunk. that is where i will attach a hose nipple for the vent, then run it to the outside under the car or out the wheel. now look at the hinge to be sure the casting isn't self destructing.

Fred