Fuel tank venting??
that looks like a one-way valve. Maybe it only allows air into the tank as the fuel is used up; check it out.
I would try to mount it up on the rear cross member between the shocks.
Oh crap, if I had bothered to click on the link,lol.........
This pic shows the valve mounted on the above mentioned crossmember, NOT in the trunk.
But I fail to see that as being higher than the filler.
Nor do I understand why that would be important. Perhaps someone smarter than me will chime in, hang tuff.
Funny thing is; the instructions teach not to put a low-spot in the line, yet the pic clearly shows one.
Furthermore, my Barracuda has a vent line that follows up the side of the fillerpipe and returns into it, a few inches below the top of it. This would be a perfect place to mount that valve..... if it wasn't inside the trunk.
If your car has a vapor canister under the hood, and if you are not using it, this would also be a good place to mount it. Just don't Tee into it, cuz that would defeat the purpose of the valve.
Like has been said; don't put it in the trunk.It will fill up with highly flammable vapor and is a ticking time-bomb.