For this you'll need:
1. Gopro cover lens replacement kit from ebay
2. A bottle cap that would fit the lens.
3. some tape for the grip, mine's duct tape
4. saw for cutting
**NOTE: The lens I bought was ok, thick and seems tough, but reduced a few degrees off the angle of view. See sample pics in the end
** I would have preffered a black bottle cap, but I only had the blue cap that fits perfectly.
Step 1: Cut the Bottle Cap
Check if it fits or if you need to cut more.
Step 2: Glue the Lens
I used wood glue, but you could use stronger adhesives if you like. I tried to use the rubber that came in with the lens. But the internal diameter was thick, and I can't push it further back. Because of that it vignettes. So I didn't use it.
Step 3: Add Tape to the Inside. This Serves As the Grip
Step 4: Some Finishing Touches.
Aftet that, finish! See the 3 pics for comparison of with and without the cover lens