First I don't "lock up" my generator fuel cans. Maybe a mistake, but nobody has bothered them yet.
I have an old milk crate, 6 1-gallon size, that I have tied into the bed of my truck. They easily fit either a 5 gal fuel can or 2 - 2 gal can. I prefer the 2 gal size since most trips I only use a couple gallons of generator fuel and this way I don't have fuel sitting around getting stale. On long trips I'll pack 2 crates, 1 for a 5 gal diesel can and the other for generator fuel. The crate keeps things from sliding and possible creating static electricity.
I guess if I was really worried about theft I'ld run a heavy cable and lock through the tanks. Same as I do for the generator.