The pictures will show what a sweet hack this is. I'd wanted both a sun shield and a macro lens, and when I got this new battery and the case that came with it I saw the possibilities right away.

First I eyeballed the length and width of the slider that I would need (about 9/16" by 7/16" )
Then I carved a channel the thickness of my slider in the large lower lip.
The opposing channel was constructed from three layers of the same black plastic sheet I used for the slider.
Cleaned out any excess glue from the track, checking it with the slider by hand.
Then I temporarily spot glued the three layer channel to the case with the slider in place to see if I needed to make any adjustments to the width of the slider.
After making adjustments I glued the three layer channel to the case with the slider in place.
Now I was ready to make the hole for my macro lens (You want to make it centered with respect to the camera) I just eyeballed it. Made a small hole with a drill, then with a round needle file made it as concentric with the hole in the case for the camera as I could by hand.
Having stripped numerous machines and devices over the years I had a little selection of small lenses.
I chose a small magnifier of a size a little bit larger than the hole I'd filed in the slider and glued it in place.
When that dried I put the case back on the phone and I moved the slider to the macro position and made sure it was centered correctly. Then a sliver of the same black plastic sheet was carefully glued in the channel as a block to stop the slider at that position.

That was pretty much it. I used 5 minute clear epoxy which allowed me to clean out glue that was in bad places while it was still tacky.