Having recently bought the one plus 7 pro I needed to update my car holder for it. I have the brodit mount so this is designed to screw on to that and leave the pivot screw accessible in the centre.
I've uploaded two designs, I have an anker bluetood module that I use so I put a side plate on mine that I could stick that to. But I've also uploaded a plain variant without it.
Update: I painted mine and then or it in the car, but what started as a snug fit became just too tight. So I've uploaded a V2 file with a more generous space which is a little easier to use
Oh, I also reduced the depth of the USB-c connector hole so that it protrudes enough to engage fully including the gap introduced by the case.
update -05-07-2019 - I've uploaded a version without the side mount that has been (in theory) adjusted for a fit without the protective case on the phone (I've not printed it to check yet) - I've also uploaded a small cross section of that fit so you can print something small to check the fit before comitting to the whole object.
I printed this in white pla, then painted/sanded/painted a bit with chaos black and finished with a clearcoat. Just to make it look a little less 3d printed and black to fit the interior of the car better.
I was actually a little inspired by: https://www.thingiverse.com/thing:3650283
this saved me measuing things up myself too much. after I printed that thing for my desk and seeing it fit well, I used the stl as a reference to draw my own profile sketch from, and took some inspiration from the design for the usb cable being threaded into place.
modeled in fusion 360. nothing special, took about 20 minutes.