Fits iPhone 6 and 7 (and maybe others). Not Plus sized phones, although the Onshape design can be forked/customized. It is also designed for these specific suction cups, if you have similar cups you may need to customize the OnShape design.

https://cad.onshape.com/documents/cc412b71b65b45ace29b0061/w/c58a897a88f69a308bcc086c/e/293b4d0ea582263a479c78ae

Suction cups:
https://www.amazon.com/gp/product/B01LQDI1WS/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1

Instructions:
1) Print the part
2) Use pliers to insert two suction cups (will be tight fit so they don't fall out)
3) Suction it to your shower on a flat surface. Put a dab of hair conditioner in each cup so the seal holds is strong, you don't want your phone falling off the wall.