Where can you find a riser that allows you to store two rows of softcover books on one shelf?

I have tons of books... mostly soft-cover books. A bookshelf with only one row of books wastes a ton of space. Where can you find an INEXPENSIVE riser that is at least 4 inches deep and a few inches high that will allow you to store two rows of soft-cover books with the ability to see each row?

Where can you find a riser that allows you to store two rows of softcover books on one shelf?
I have successfully used styrofoam for EXACTLY this purpose! You just cut it to size. Foam core might also work -- or you can just fold up some cardboard to fit. It won't show!
Reply:Try putting a concrete block on each end with a 2x 4 or 2x6 across it. Its amazing how well it works and how cheap it is.