I recently upgraded my phone to a OnePlus 8T, and so I needed a case for it. This is the case I created for it, designed to be printed with flexible filament. As with my OnePlus 3 case, I printed this with no bottom layers and 20% gyroid infill to give it a cool and grippy surface. 

I must make an important note that if the back is printed solid, the shock from a drop is not dampened and the glass on the back of the phone can crack. This happened to me while I was experimenting with case designs. Make sure the back surface of the case is not solid infill!

This case was designed with a minimum wall thickness of 2mm. On average, the walls are ~2.3mm thick.

This part can be printed without supports, but using supports can lead to slightly better bridges (if you can remove the supports cleanly).

Update:

I modified the original CAD to have 3mm walls. I have now added the alternate 3mm version to the file list. The benefit is better drop resistance, but for some added bulk. I also modified this model to grip the phone tighter, at the expense of the case encroaching around the screen more. I personally prefer this model and feel more secure using it, despite its extra size. 

Link to Original CAD on Onshape