A big no on using a seat pan to mount a fuel cell. It's not designed to hold that kind of weight pulling UP when you crash, and they almost always let go and jettison, sometimes right into the back of the rider's head. Remove the seat, make a plate to BOLT down there, mount the cell to that.
Yes, all fuel cells need a vent or they won't flow. Some use a combo roll over valve/vent, others just use the existing hole in the tank at the highest point, (don't forget side stand tilt), and run a hose down to the back of the bike. Often angling off from the rear fender so it won't splash on the tire.
No, vented caps don't prevent gas from leaking out. Unless they are submerged, there isn't much chance of water getting in because the vent is not directly exposed. (edit - I see some of the ones you linked to have an exposed hole. If you look at them you will see it's nothing more than a hole thru the aluminum cap with a brass/bronze screen filter on the inside.)
The reason vented caps are not allowed, (and it's a bad idea for a moto), is it's very easy to fill the aux tank, ride off on a hot day and have the cold gas from the underground service station bulk tank start to warm up and expand. Now it's spraying or leaking out of that vented cap, and running all over the bike. A particular incident involving a Aprilla Caponord and a fire while riding to the Ancient Bristlecone Pine Forest comes to mind. It was a good day when the rider didn't burn down one of the oldest trees on the planet.
Yes, all fuel cells need a vent or they won't flow. Some use a combo roll over valve/vent, others just use the existing hole in the tank at the highest point, (don't forget side stand tilt), and run a hose down to the back of the bike. Often angling off from the rear fender so it won't splash on the tire.
No, vented caps don't prevent gas from leaking out. Unless they are submerged, there isn't much chance of water getting in because the vent is not directly exposed. (edit - I see some of the ones you linked to have an exposed hole. If you look at them you will see it's nothing more than a hole thru the aluminum cap with a brass/bronze screen filter on the inside.)
The reason vented caps are not allowed, (and it's a bad idea for a moto), is it's very easy to fill the aux tank, ride off on a hot day and have the cold gas from the underground service station bulk tank start to warm up and expand. Now it's spraying or leaking out of that vented cap, and running all over the bike. A particular incident involving a Aprilla Caponord and a fire while riding to the Ancient Bristlecone Pine Forest comes to mind. It was a good day when the rider didn't burn down one of the oldest trees on the planet.
So, if I'm tracking this properly, I can buy the one I originally listed (here for reference) and apply one way vent to allow air in, but not out like so?