I forgot to show this last week. I sorted all these books and doubled them up. For the bigger books, the riser is a piece of square ABS piping cut to size. For the paperbacks, I got Variera shelf inserts at Ikea, normally used for kitchen cubbards. They are a smidge smaller than the Kallax cubby, so I had to make sure I put a wider book at one end. And voilà!