Okay, I’ve used these enough times now to think they might do someone else some good.
If you use RAM mounts the 
will let you design great add-ons quickly for phones and tablets (or whatever) that they don’t have mounts for. I’ve done this at least three times already.
I’m not providing plans for a particular holder as everyone’s use case is different but here are the parts to mount the ball joint mount from RAM.
Cut one of the ones with the round holes and as many as needed to give you clearance with the hex.
Glue as appropriate for the material being used.
Here is the latest one: