A-HA!!! GOTCHA!!! Didn't even think about that. So that cap on the clear jugs is just a cap, no funnel neck, no rubber hose... just a lid. Awesome! Now I've just gotta find a decent cycle shop nearby to see if they carry 'em.
Yessir! Tuff Jug even send's them with a handy dandy "rubber" gasket that "should" seal that cap up nice and tight
(I've had mine full of gas or E85 or over two years now, and while there is some discoloration around the mouth of the bottle, the gasket is still in like new condition. If they were truly only rubber, they'd have been eaten up a few months after the first gas went in. Just take a quick look each time you fill the can, and make sure they're still in good shape.)
Another quick bit of advice: don't bother popping a hole in the breather or removing the plug from the cap. You'll see what I mean. There's no need to pop the breather because you'll be removing the cap when using them.
By the way, is this the first time you've stored gas? If so, consider...
-gasoline expands with temperature. Those Tuff Jugs are tough, but if you leave them in some heat, they'll balloon out. I've never had one fail, but its kind of annoying since they roll some on the balloon rounded bottom instead of staying put on their normally flat bottom. Still though, these Tuff Jugs hold their shape better than any other can I've tried to date.
-you need to rotate stored gas often. I wouldn't go more than 1 month without using it. Some say you can go up to 6mo if its tightly sealed to keep water out. You make the call.
-it stinks. Ohhhhh does it stink. Even with everything sealed up and no leaks, you can tell I'm carrying gas from outside the truck. If you're going to carry food and those gas cans in the same location (bed of the truck?) make very sure all food is tightly sealed. We ruined an entire cooler's worth of food once because we didn't seal everything in ziploc bags; all the food ended up tasting like gas.
Just some stuff to know, so you're not worried or surprised. I don't carry gas anymore because of these reasons (I carry E85 instead which travels sooooo much better, but I realize that's not an option for ya
)