Cardboard cover, your truck had one when it was new. I bought mine at Classic Parts.
use promotional code "Stovebolt" and you'll get 5% off your order. It's shipped rolled up. after you unroll it, it takes a day or so to flatten out. You have to slice the ends and fold (you'll see the marks) and use a staple gun. The ones you use installing insulation T-50. This shapes the ends to fit the tank. The flaps at the top slide into slots in the cab. You really have to take your time with it so you don't crease the cardboard.
http://www.classicparts.com/1960-66-.../#.UikkQ_PD_IU