Elements with 2 or 3 hooks mountable to Billy shelves without screws or any tools. Usable for hanging jackets. it uses the slight overhang of the side pieces on the top for stability. The design has sufficient space for two sided tape for added stability (optional).

Best printed with supports on build plate and hook tips pointing towards the build plate. 

If you are looking for a single, strong hook, consider this: https://www.printables.com/model/934370-bag-hook-for-ikea-billy-no-screws