The "universal" phone holder in my 2012 Mini Countryman wont hold the longer iPhone 5 so I made this one. It's slightly larger to account for the additional size of most cases. I've been using it and while it's not the same quality as the original, it work very well.
The flat base is 3mm and quite flexible. It doesn't bother me but if you want something more solid, go w/ the 4mm base. Also, a shiny plastic material might be better since the Strong and Flexible (Black) marks up very easily.
You'll need a torx T10, T25, and a large flathead screwdriver or knife to take the original phone holder off the Mini rail.