[travelin texans]

Most hardware stores sell small pieces of aluminum plate, as mentioned use about a 4x4" piece, or whatever it takes to get a few screws around it & Gorilla glue behind so it holds into the plastic interior of the door.
If needed a piece to cover the entire bottom few inches of the door interior so you could attach it more solid to the door frame work. A piece of aluminum diamond plate on the bottom of the door wouldn't look bad either & would make door more solid.