What material works well for speaker mounts?

I am helping out my nephew install new Boston Acoustics 9in speakers in his truck, and we have no place to put them. I figure on fabricating something to hold the speakers in his truck door, but don't know what materials work best for sound quality. The chore wouldn't be so bad but putting them in the door means they can't go all the way in or the windows don't roll down. There is really no room on the dash for any alterations to put the speakers there. Any ideas are great!!!