The shower shelf has various openings for water and air circulation. One of the legs has an angle at the bottom to compensate for the tilt of the existing shower shelf. At the bottom of the legs there are small "socks" made of TPU to create a full contact area with the clamping pressure of the cable ties and to protect the existing shower shelf.

I'd printed it in two parts (shelf with legs + railing) and glued it together.

It was nearly my first try with Fusion 360, so be aware with the fusion file. It's completely messed up. I'd added it just for the sake of completeness.