Years ago, I made a plywood box to store 2 each of 1-gallon metal cans of extra gasoline.  Mounted under van, in slider door area.  I typically exchange old fuel with new stuff; today was the day.  Well, in each of those cans....only half remained after 2 years.  I had investigated super duper gasoline-capable rubber grommet material, and had installed under each screw cap.  No good, it gently leaks out.  So...what to do?  In previous decades, I kept the old school metal gallon can (like the kind you may use for your lawnmower) on a shelf, approximately at the location of the rear heater, long deleted.  I figured that was about the safest location for explosive fuel; best for front-end or rear-end collision, adequate for side collision, super easy to get to.  I was worried that I was not being smart.
But, I am re-thinking all of this.  Why do I need extra gasoline?  Yes...for me....and a stranded motorist, that was the intent.  This has NOT happened in 30+ years of Vanagon ownership.  Also....I am pretty good about planning ahead....and keeping an eye on the instrument cluster gauges.  Also, Subie 2.2 delivers a reliable 22 MPG, so I'm good for 250 miles, even coaxing 300 miles.
Am I overthinking all of this?  Probably.....the best thing to have....is a cardboard sign that says "I NEED GAS!"....an appropriate length of Tygon 3/8" tubing for siphoning, some kind of empty container, a $1 slim Walmart funnel....and hope for the best that some kind person will stop to help.  And, a nice $20 bill for their kindness.

No....I don't want exterior Jerry cans and such hanging off the rear end.  Also, I plan ahead for whatever short off-road trips I do, including fuel and food and water.
Anyway.....just tossing this out there to the List....perhaps you've come up with some cool idea regarding carrying extra gasoline?
Rich
San Diego