It would be easiest to grab one from the JY, but if you're handy and have some time to kill, you can make your own. I've been doing automotive upholstery for 25 years and have used different things over the years. The easiest to work with, when you have to get it to mold around a curved section is some 1/8" particle hardboard from Menards or Lowes or someplace like that. The foam backing I always get from my suppliers, but you can order 1/4" foam from an online upholstery supply place. Then just grab some material you like... make sure it has a good UV colorfast rating or it will fade within a few weeks.
Glue it all together using a spray glue designed for headliners. You need something that will hold up to high heat. That's another product I get from my upholstery suppliers, but you can find it in rattle cans at the auto parts stores. Just make sure it is designed for headliners. Regular craft or foam spray glue will fail quickly when the summer sun heats things up.
