Originally Posted by Last Tango
There are inboard tank caps that incorporate a one-way valve for venting. (not sure if that is compulsory in the USA for emission regs.....
On cars they're mandatory (in Calif, a car will fail the state emissions test if the fuel cap doesn't seal.) Don't think they're mandatory on boats, but I also don't know about requirements for inboards. I'm pretty sure that the vent on my built-in tank (30 gallon, feeding an F-115) is just a pipe to a hooded vent; no valving.
(I've heard they don't get out of bed until the middle of the afternoon like our teenagers)
Perhaps, but we don't do as much soccer hooliganism either.
BTW, to the OP, I'd agree with 9D280 about going with 3D printing. Cheaper and faster than about any other solution for small quantities (unless you know a good machinist who will donate their time.)