I had one for a Toyota years ago, it was made by Leer. It was aluminum, had a ladder rack and long side doors for access to some bins I built in. Cost about $400 in 1988. I wanted a fiberglass model for looks, but was told they couldn't make one with the ladder rack. I have seen someone with a ladder rack built around the cap.
The cap I had was put on with 4 or 6 #10 screws and was easy to remove.