I had a cover on a '48. It was just the material that an upholstery shop sewed up. Sometimes boat shops make custom covers for boats as well. It was a basic square pattern with the edges sewn over. Snaps were screwed in place on the rim of the bed. A 2x4 was cut to fit (notched) in the middle holes across the bed, and then the top of it cut off in a rounded shape. I thought it was very simple and nice looking. Here is a picture of the truck.

http://www.stovebolt.com/gallery/vandenberg_steve_1948.html