I used the Reflectix as well, plus some carpet padding and then covered it all with some nice Redwood and Cedar I had laying around. My interior has a lot of wood so it works nice. I did however move my gas tank out of my cab for more storage space so you may have a little less room to work with...
Pics can be seen on page 6 of my build thread (click the link in my sig) just scroll down a couple.
I would think thin sheet metal or masonite as suggested by Chev4t6 would be great and simple ways to go as well.
I find I really only notice the upper section if the seat back is in position, and if your gas tank is still there id think you wouldn't be going back there that much. Could always throw the seat and frame back in quick to get an idea maybe..
Dunno if that helped or not
Chooglin
