Joe,
Recommend you do as another poster here suggested...which I also did. Buy some speakers and speaker boxes and just sit on the shelf or seat, whichever you have.
FYI, I found some good speaker boxes and Pioneer speakers at
http://www.etronics.com.
When I want to use the back seat for grandkids, luggage, etc., I just disconnect the speakers and store them or put them in the trunk.
Wish I could remember who made this suggestion so I could give appropriate credit, but I can't.