This is a phone holder for my VW caddy, to put on the ventilation grid next to the steering wheel. There's a handle in the middle of this grid that makes it difficult to attach things so I left a cavity to fit around it (in the part "connection.stl") and it does so perfectly. I don't know about other cars though. Maybe you can use it as a starting point and edit it. The reason I put no rim on the lower left side is because I have a phone sleeve with a cover which needs to be open to navigate.

My holder also requires a 2mm or 3mm thick magnet (I am a fan of supermagnete.com) to be glued on the thinner part, to keep the phone in place. Both my wife and I have metal on the backside of our phones but you could also use some magnetic strip. It doesn't have to be super strong as the magnet does not carry the weight of the phone.

I printed the 4 loose parts separately with the flat side of each part down (separate stl files), and glued them together. The whole thing is angled 20° from a vertical axis (see stl of complete item).

Initially I forgot to make a hole for charging and the rim at the front was too high to reach the bottom of the touch screen.I also made it narrower because it just looked to big. I didn't print the edited version yet.