Bathing, in general, is a process that is much more satisfying with the proper application of a towel. Think about it. Whether it is a bath, a shower, or that cool method that they use on the International Space Station (https://airandspace.si.edu/stories/editorial/how-shower-space - which can't really be called a shower, because showers are, by definition, falling water), towels really do complete the process. Without those towels, we would just have a lot of soggy, and therefore cranky, people walking around everywhere.
I needed to install some towel hooks in my shower, but I didn't want to drill into the faux marble shower walls.
I started with sergep's "Customizable U-Hook" (https://www.thingiverse.com/thing:1367661). He has a really useful customizer tool for wall hooks and over-the-door hooks, but it didn't seem to do exactly what I needed. I took the hook portion of one of his sample hooks and custom designed the top portion to attach to the wall above the shower. My shower walls are 1 inch (25mm) thick. The trim around the shower is 4 inches (100mm) wide. I designed this hook to hang at the bottom of that trim piece.
I attached it to the wall with a weather-resistant screw and exterior-grade, waterproof double-sided tape from 3M on the top of the shower trim. It is pretty sturdy.
This design works for me; maybe it will work for you.
I used PETG for strength. Clear PETG is really pretty.