Step 1: Parts list
A hard phone case
A tripod mount from a scrapped camera or a 1/4-20 nut from a hardware store (1/4" 20 thread count)
Polymorph (plastic granules which melt in hot water).
Very hot water
Step 2: Heating the Polymorph
all clump together. You need to remove the molten Polymorph from the hot water and squeeze the remaining water out. CAUTION! It will be hot to the touch.
Then you need to mould it around your tripod mount/nut.
As it cools the polymorph will become more opaque and harder to work but resubmerging it in hot water will soften it up. You may need to refresh your hot water if it cools too much.
Step 3: Moulding the mount
Step 4: A note on position
I'd postition the mount a little further back allowing the phone to stand on its side when I'm not using the tripod. There's been several instances where this would have been useful so it is worth considering.
Step 5: Aesthetics
Keep all your offcuts. It can be all be reused by reheating it.