Anything with enough strength to support the weight of groceries/random things that get tossed in the trunk is going to add weight itself. The whole point of taking out the spare is weight reduction. IF I were to use a metal or acrylic copy of the cardboard piece already in there, it would have to be mounted in a way that it wouldn't slide around. Which would defeat the purpose of using it to keep the empty space open. A form fitted piece of foam will be very light weight and will support any trunk cargo without any fabrication. Think of it as a "no hardware required" installation. If I ever need to put the spare back in, all I have to do is swap the two out. So with the points of 1. Price, 2. Weight, and 3. Ease of Install , this is the best option for me and possibly others.