I finally decided to use the foam boards for my fabric and have spent most of the weekend working on that project (there might be a teeny, tiny little chance that I have too much fabric!).
I bought the boards at Dollar Tree and cut to size (quick and easy with a rotary cutter and T square!). My shelves are also 20" deep and I could do 2 rows of mini bolts. Right now I'm thinking I'll use the space in front for fat quarter storage - as soon as I decide what to put them in, or for WIPs.
I also like the idea of storing batting packages behind the fabric. The fabric is easy to move on the boards so it won't be hard to move it out of the way to get to the batting.
I was concerned the bolts would take more space than the totes I had the fabric in previously, but they don't. I definitely need more light in my closet though.....the better to see all my pretty fabric :-)