My bathroom includes two simple towel racks right inside the shower, but not enough storage for things like sponges, shampoo bottles, etc.

This design provides a clip-on shelf that attaches to the simple dowel-based towel rack. The shelf has drainage, and is modular so you can install the clips before getting the shelf in the way.

OnShape (free online CAD tool) design is public here

• I designed this for my 22mm diameter towel rack dowels
• In the design (linked above) you can modify the dowel diameter and distance from the wall (size of bracket holding the dowel) by adjusting the distance constraints in the “profile of rack” sketch (the first design element listed)