iPhone holder for Roadster or Phaeton?

[David in San Antonio]

If you have a Roadster or Phaeton have you found a way to attach an iPhone without drilling any holes?
The bracket sold by Snyder’s is for closed cars. The Roadster interior sheet metal at the base of the windshield is different (no channel to send air to the floor).
I tried a suction cup vacuum mount but it lets go at odd intervals and leaves round marks on the paint. The round marks can be cleaned off but that paint is 65 years old already. Also tried neodymium rare-earth magnets to stick to both the dash (with microfiber cloth to protect the paint) and to the thin ferrous plate I attached to the back of the phone, but even with several magnets the phone slipped off.
If it’s any help: I’m also going to rig up a USB port to keep the phone charged.
By the way, the phone is just used for its GPS application. The noise in the Wretched Roadster makes speakerphone conversations impossible anyway.
Your thoughts?