Well, one solution would be to remove your current filler neck and replace it with a vented one, but don't install it in the normal location, have it point up into the trunk. Run a rubber hose from the vent tube on the neck thru a hole in the floor of the trunk to the outside of the vehicle, you might also try putting a loop in the vertical plane lower down on the hose, use a zip tie to secure. Install a non-vented cap on the new neck. If you have a conventional filler cap, cut your old filler neck off the part that secures to the rear panel and maybe even block it off by bending the old tube over or weld a piece of steel over the opening. This will keep unscrupulous individuals from putting stuff into your trunk via the hole.
This will allow you to have a "locking gas cap", you won't have to get on your knees, it'll fill easier, you won't have the dreaded gas running down the back of your car when you "accelerate spiritedly" and you shouldn't have any interior fumes.
Only drawback is you need to take care fueling the car up so you don't get gas in the trunk. If you have access to a welder, you could weld a cone of metal to the new filler neck and add another tube that vents to the outside, this one without a loop. That way if you spill any gas, it'll go outside the trunk.
If you're wanting to relocate the filler, you'll have quite a bit more fabrication to do.
