The best bet is removing the pour spout and using an adequate funnel. The problem is in transporting the darned things in a vehicle. I finally have just tossed the spouts and use home made RTV gaskets on the cans for transport
LOL, when ever I make a run to the filling station, I always park my truck next to the water hose to wash out the bed when I get home.
Bed liners don't like diesel or gasoline.
Wouldn't even think about putting them in the trunk of my car.