Fuel Can Seal Cap

Have you ever gone to fill up your fuel cans and wound up losing the black plastic seal cap that fits above the nozzle when it is inverted in the tank and below the threaded cap? I own a small ranch, and constantly need to use my fuel cans. As such, I lose these little buggers all the time! Now, the cans should seal just fine as long as you leave the nozzle out in the "fill" position rather than stowed away in the "storage" position, but this is not always the case. I can't tell you how many tim