For a pretty long time (at least the 50's through the 80's), it was popular for showers and bathtubs to include a distinctive towel rack on the wall opposite the spigot, with a square towel rod. I've never found them particularly useful, so I designed a shelf to mount to them instead.
In the showers in my house, the distance between the ceramic mounts (where they attach to the wall) is exactly 47cm, so that's how wide I made the shelf. I'm not sure how standard that distance is, so I've also created a 37.6cm-wide version. Also, some towel racks have removable square dowels that you can slide things onto and re-mount, while others have permanently-installed dowels (which is, I think, the more popular style). To accommodate these two styles, I've created both slide-on and snap-on versions of both sizes.
The width is parameteric, so you should be able to adjust the SCAD file to exactly fit your shower.
Finally, I've designed everything to fit in a 120x120x120cm cube, so as to make it printable on an MP Mini Select. Each STL file contains four separate segments that you can slide or snap onto the towel rack next to each other to produce a full shelf.
One minor warning: while there's not a lot of plastic used for these, the design is somewhat intricate, and so it can take a while to print. When using PLA on my printer, I've found that it takes between 17 and 18 hours for a complete print.