This was the first model that I managed to print using the warping white PLA, because the outer corners are rounded for a better distribution of heat (nothing sticking out) and tension (pulls sideways when shrinking, not inwards). Used 0.1 mm layer thickness and sanded by hand. The ugly marks are probably from a slight over-extrusion at 100% infill and my useless spool of white PLA tangling up into knots all the time. An 80% infill and not letting the nozzle jump over gaps (leaving lots of strings) would probably solve most of the problems I had with white PLA. Would be a lot easier with gray PLA, but one can also give it a few layers of car paint to hide the marks. To mount the holder on a wall or furniture, one can tie it up using threads, tape through the holes, glue on a mounting device, or modify the model's back to match your furniture before printing. If you need to adjust for your own phone model, you can import the STL model in Blender, move vertices around and upload a modified version linking to this design.