This is a simple wall tube holder I made for my father-in-law's CPAP machine which uses a 20mm diameter tube.

The original size is 75mm tall with an inner diameter of 35mm and an a top opening of 25mm. I included a Fusion 360 F3D file, so you can adapt it to your needs.

Ideally this would be mounted with 2-sided tape so that no screws are required that could damage the tube. But if that's not secure enough and the model is sliced with enough inner walls so they touch at the back plate (see screenshot), it should be good enough to drill holes for screws.