I am not a fan of carrying around gas in one's car in this way. The gas in your car's gas tank is about as well protected as it can be during from collision damage. Any gas can kept in a trunk will get squashed in a collision and leak causing an extreme fire hazard at a very inopportune moment.

None of the currently available gas cans (at least in the US) are worth a hill of beans IMO. Some lunatic US federal government agency enacted idiotic regulations about gas cans that are all but impossible to accomplish with a gas can that will actually work well, so basically all gas cans currently sold in the US are all but useless.