vented gas cap for '56

[mark '87 930]

Hello everyone, for those of you who have converted your stock, behind the seat tank with an EFI setup... how did you vent the tank?
I am thinking of going EFI AND will retain the stick tank. I have a couple of options for return lines (will likely go with the FItech system with their FCC or the one from Rob), either through the sender or via the filler tube.

My question is, what do you guys do about a vented gas cap? With the return line, there will undoubtedly be pressure buildup of sorts, i don't want the cab smelling of fuel (for obvious reasons), so what do you guys do? I haven't been able to find one for the stock setup. It's not like it's a great seal with that cap anyhow, but thought i'd ask.