For touring and not racing, I went a slightly different route for my 9800 km drive in my S3 from Colorado USA to Alaska, two months ago.
I had a removable "jerry can" constructed of aluminium, somewhat triangular in profile, and designed to fit in the boot between the shock towers. It has a removable filler cap and a vent cap (loosened during filling), and a one-way vent with a snap coupling (outboard motorboat fuel fitting) that is hooked to a vent hose installed in the car, to vent any build-up of pressure while driving.
The jerry can is held in place between the shock towers by two safety-belt straps and scuba-diving buckles, which pass through metal loops (taken from car seat belt shoulder strap keepers) that are bolted to the underside of longer versions of the Caterham shoulder harness bolts.
When my main tank gets low on fuel, I stop, remove the jerry can from the boot, and re-fill the main tank. The jerry can holds 4.4 gallons (US), and weighs 11 pounds, empty.
Alaskossie