I often travel with 2 x 10 litre plastic fuel cans strapped down in the cockpit, I fill them leaving a small air pocket then compress the cans with my knees till fuel is nearly at top and seal the screw tops
Not advising anyone to do this, but my aircraft's main fuel tanks are 2 x 30 litre plastic containers placed directly behind the 2 seats.
I worry more about the 2 fitted tanks than the spare fuel cans.
P.S. I believe its actually Illegal