Although there's tons of iPhone car holders in the market, for the life of me I could not find a decent one that mounts on the car vent. I also could not find any that mounted the phone in landscape at that location. I had a generic one for my Android phone before switching, and really liked the mounting type, so I kept using that for a while, even though it was quite bulky and I had to manually insert the Lightling cable. Once I got a 3D printer, I decided to make my own, that satisfied my requirements (vent mount, landscape, compact, holds the cable so insertion is a snap). Since this would be substantial effort, decided to invest some little extra time to make it parametric, so others could customize it for their own cars, cables, and phone cases.
Well, here it is! Hope you find it useful; features, in no particular order:
* Reasonably slim, landscape.
* Cable and plug channel, so insertion is fast and easy.
* Two mounting options: vent slats or CD slot.
* Selectable cable channel profiles.
* Customizable dimensions to accomodate different cases.
* Design that uses sliding clips to allow adjusting fit, without re-doing large, tricky prints (the main holder body); should support any car vent (as long as your slats are horizontal).
* Does not cover headphone jack or ambient light intensity sensor (although you can optionally change latter, if you dont like the cutout, but then you may have to manually adjust brightness, unless you always keep it at high).
Note: The pictures are from an earlier print with full-height front, which covers the light intensity meter. I've since switched to the current design, which leaves that free (although old design is still an option).
I have tested this with the cases and cables I have: a Ringke Slim case (which I really like and have on all the time), an RND 6ft cable (which is long enough to reach the lighter plug in the armrest) and the OEM lighting cable, and of course my own car. If you customize for different cases and cars, please post in comments if you find issues. Customizing the cable profile needs adjusting OpenSCAD code (cannot be done with numeric parameters), so you'll have to download the source -- please share the modifications if you do.
I've been using this for close to two months, with no issues (we'll see when winter comes and I need the heater, but I'm guessing it should be fine :) ).
**Update 8/25:** Added CD slot mounting option. I still prefer the vent mount, but a few people suggested this. This is experimental, not thoroughly nested for strength yet (this configuration has been in my car for just a few days).