Very small towel hook, designed to be attached with double sided tape. I couldn't find one like this so I made it.

Strong double sided tape will have absolutely no problems holding one towel. Most models used screw - and it does not seem like a good idea to put the screw thru tiles ;)

Print on the side (just like the model is exported). I used PETG with 15% infill but it doesn't really matter, basically anything will be fine.

Mine took 13 minutes to print (core one, 0.25mm structural) and used 4 grams of filament. The included STL is for 18mm wide double sided tape. You can open source in openscad, change width and export your customized STL. Model is fully parametric so you can change anything, including thickness and length.