Why?it seals. You can't lose the cap. Safer in an accident
You may want to measure the diameter of the pipe with the filler top, then go to a Home Depot, or hardware store and look at rubber pipe caps, there black with a hose clamp, if one is the right size, just remove the clamp and slip the rubber cap over the top of the filler
There is also an expandable rubber plug that can be inserted to keep crud out once you measured the id of the fill pipe to get the correct size. Hardware or plumbing supply house has them and they are cheap.
The bigger issue on the thing is the damn flaps that SLOW DOWN the filling process or take a bath in fuel oil you are paying for that now you have to clean it up and you still smell like #2 fuel oil if it splashes on you or drips on your pants / boots. Fueling should not be such a PIA. A straight fuel pipe with a cap will be sooo simple to use without the flaps slowing things down especially when it blowing like hell and the temps are in the single digits. Ram botched this aspect of the truck.