I cut a tire board out of lauan also. I'm running a 205-65/15 spare, so the board didn't fit very well anyway. My 68 had absolutely no insulation whatsoever between the interior and trunk. I went to Lowe's and bought a roll of the foil insulation. That's what I used for the entire interior. Just use 3M spray adhesive to stick it on.
I didn't want to cut up the original trunk liner so I used shower pan liner. LOL
I thought of using 1/8" hardyboard (hardieboard?) for the divider and tire board. Just ended up using what I had here available.