Billet Fuel Cap for Ram CTD?

Ram makes a plastic cap with a tether strap.

Their cap fits snug and overlaps the outside lip of the fuel fill neck. If water gets on or near the cap it should run off the outside of the fuel fill neck.

The aluminum caps look better. I have read of people needing a different size o-ring to make it fit better. Some ship with three o-rings. It really isn't a cap, it is a plug. If the o-ring does not fit snug, water that gets on the cap could still get into the fill neck. I know, there is a second flap in the fill neck to divert water, but I prefer to not depend on it.

Mopar 82214609 Non-Locking Gas Cap (Diesel)

Drill a hole in the plastic tab and tie it in with a string . Won't blow away. Easy fix.

I got one from Amazon for $15-20 and I like it a lot. I kept the little red cap for a year or so but like you say it had a tendency to blow away in the wind. This one is magnetic so it sticks to the filler door while fueling, and I really think a cap is a good idea compared to capless.

Yep. I have one.