I completely re-designed my Moroccan Shelf Riser, which I designed to fit in my IKEA Kallax bookshelf. It's now extra strong with additional supports for heavy books and movies.
 

Two of these risers, placed side by side, perfectly fit in my Kallax's cubby - there's a very small gap on each side to allow for you to slide the shelves in or out.

I wanted something aesthetically pleasing, and the right height for me to be able to read the movie or book title on the back row. I used a Moroccan print for the top, slim legs with brackets for extra-support, and middle legs on the back and front for extra stability. I tested it out by placing a very heavy box set of DVDs on it for a week, and it has shown no sagging. For extra stability, you can zip-tie or super glue two shelves together.

Some people have asked me “why a Moroccan print?” There are two reasons for that. The first is that the shape of the print adds increased strength to the shelf riser. The second is that I love Moroccan prints, and I designed this to match my existing home office decor.

If you'd like to maximize your storage, you can also print my mini baskets that slide underneath the riser.

Print it face down, and it requires no supports. If you find that it's too short, you can adjust the Z height in your slicer; I have not tested the strength and stability at other heights though. I use PLA, and I recommend printing from a dry-box if you opt for PETG.