This basic toilet paper holder is designed to hold the roll upright, while sitting on top of the tank using suction cups on the bottom. The top of the tube is flanged slightly to allow you to add and remove new and spent rolls, but ensuring the roll does not slip off during use.

This design is very basic, but functions quite well.

I printed this in white PLA+ using a Ender 3 v2 equipped with a 0.6mm nozzle at 0.3mm layer height, 25% cubic infill and 2 walls. I recommend using ‘Bridge Mode’ for the overhangs, not supports.