I recently bought a TechMatte magnetic vent mounted phone holder (https://www.amazon.com/gp/product/B019GL78NK/ref=od_aui_detailpages00?ie=UTF8&psc=1). It works well, but I wasn't crazy about the vent mount part.

I designed this piece to re-use the magnetic part of the mount, but allow me to mount it to a flat part of my dashboard with two sided tape. The magnetic part of the mount fits over the ball perfectly, even with a coat of primer and two coats of paint on it.

I also designed this piece to work with 3M command strips. I have this attached to my dashboard with a 3M command strip and it works great. It has been holding up for months.

Make sure you use the latest file with the beefed up main shaft. I found that the weight of the mount itself caused the piece to fail in a hot car. The revised design seems to work a lot better.