Lud, been thinking about this for years. Like WW2 Allied and Axis (Asian aircraft did not use em) Fuel bladders.
They collapsed as the fuel emptied lessening the need for venting. Could be fitted into any shape or faux tank. And for those late night rides into the north end of Hartford, won't burst into flames when shot.
It has been a couple of years but did research it. You can buy the material and make one but surprisingly enough, is pricy. So was gonna experiment on some sort of jogger's or bicyclist's hydration bag. Would come with a "filler" and tubed outlet. And some sort of shutoff.
Every type of plastic you should not put gas in that I could find, I've put gas in. They have held gas and not broken down. I kept gas in a laundry detergent container for over a year and was fine. Just saying if anyone tries this, make sure gas will not pour on to your very hot engine and exhaust. Makes for an interesting day. LOL, don't ask how I know.
I made up that last part. (Was in no way "interesting" snork)