I used those gas cans in 20 degree weather this year. Getting the piece to notch on the rim of the generator, finding the other catch, then letting it slowly glug into the generator while freezing my buns off... was not fun. Older gas cans at least had a vent hole so it didn't splash all over the place. New cans, one has to be very careful and take 5-10 minutes, or else more gas would wind up on the generator than in it.
I also wonder about RotoPax. You can buy them, then buy the "water rated" spouts which from what I see, are just like the gas can spouts before the EPA came in with their heavy hand.
Of course, there are always Type 2 cans and NATO jerry cans.
My next rig, I'm looking at either the metal jerry cans or the RotoPax containers. Either of those can easily be locked to the vehicle, trailer, or cargo carrier via the mounting system.