On my cars (unfortunately no Kaisers or Willys) I like to use conveyor belt. It is reinforced with some sort of fabric, and available in different thicknesses. About 1/4" works well in the cars I do (Studebakers). A lot of industrial type places use conveyor belts, ask your friends and find someone who works at one, and can get you scraps.
Interesting fact, the original body to frame cushions on Studebakers were cut up old tires! The ones with part of the whitewall are the coolest.