A hockey puck is also something that could be cut to fit the pockets on the sides of the car.
If somebody wants to make some inserts and doesn't have access to woodworking tools, one tool you might want to consider is a vibrating saw. Fein's patents started expiring so there are lots of inexpensive saws. I have one by harbor Freight that was no more than $20, might have been as low as $10. It works fine. An advantage of a vibrating saw is it will not cut you. Your flesh will move so the cutting action won't work. You could jam the saw against yourself hard enough to do a little damage but the saw won't cut you if you are cutting a hockey puck to the correct size and touch your finger with it. These saws are also nice for cutting drywall and other miscellaneous tasks around the house. This is the one I use:
128i Convertible, MT, Alpine White, Black Top, Taupe Leatherette, Walnut, Sport
Ordered 5/22/09, Completed 6/4/09, At Port 6/9/09, On the Georgia Highway 6/13/09, Ship Arrived Charleston 6/24/09 at 10pm, PCD 7/21/09