This is just a cheap phone holder setup I put together a few days back and thought I would share in case others wanted this info. This was made from the Onn universal phone holder kit from Wal-Mart for $10. Onn Phone Holder Kit
The kit comes with quite a few pieces but you'll only need the two circled in red in the first pic.
Here are a couple of pics of how the finished pieces look installed:
Here are the two pieces you need from the kit:
This is a closeup of the piece that needs modified. Cut the ball and stem off the end of this piece as close to the base as possible. After the ball and stem are cut off, drill a hole big enough for the bolt (next picture) to fit through. Drill down through where the ball and stem was attached.
Here is the bolt size I used. It is a bit too long, but it worked. (It's an M6 1x20. A 1x15 would have been sufficient.)
This setup works just fine for me. Note that I already had the aluminum window support in the Slingshot. It was in there when I bought it used.