I've made a High-Level El Capitan with all Lionel Cars.
What I didn't like was that there was not a cowling on the crew car next to the Step Up High-Level car. I know that K-Line and 3rd Rail made one, but not Lionel. So I modeled my own. I'll be putting the models in the download section.
This one fits the Lionel and Williams 18" cars but not MTH. I attached it using thin 2-sided tape.
Model:
On the Car:
I left the bottom smooth and then used a soldering gun to melt openings for the details on the roof of the car:
Here is the final after I painted it and put on ribbing:
