I wanted a to use an inexpensive eclipse glasses filter on my phone but the rubber band method didn't give me quick access to unfiltered pictures or my phone's light. I ended up creating this simple cantilevered, swing open filter holder. It isn't robust but it only has to last an hour or so.
I used a 3mm rod as a pin but a small machine screw or even a toothpick would likely work. 3mm rod binds to the mount very tightly.
I laid out both parts so that supports aren't needed (at least my PLA bridges the gap without much sag).
I added a raft to the lens slot so it would print vertically avoiding supports.
The slot and mount are two parts to work with phones with lenses at different distances from the edge
On my wife's phone the airplane phone holder interferes with the power button so I simply use more rod to space the mount from the holder.

Creality

Ender 3

No

No

Add a inch or longer 3mm rod/ bolt/ toothpick to the mount and mate the mount and airplane phone holder of your choice. Cut out a solar eclipse glasses lens, slide into the slot and slide the slot into the mount. In my town I can buy paper solar glasses at the local hardware store and in 2017 I bought them at a gas station.

TinkerCAD for the win!