Summary

https://youtu.be/fRDuPItwpfQ

UPDATE:
I can't help myself and finally made a better one, try to use some shrinkable tubes and 3M tape to protect your phone. No support needed(already built in). I recommend 3mm brim. There are two versions of the main part. Only "8mm" can be used on most phones. Another one just for the phones less than 7mm thickness(So do not download V3mainL&R and pin.stl unless your phone is less than 7mm thickness).
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I'm sorry for this ugly and stupid thing (and my awful English off cause) It looks really sick :D

I spend a short time designing this thing. Hope it will help you.

Basically, you'll need some aluminum tape and double tape to connect the trigger and the firing pin, and two elastics. I used some ABS filament to cross holes and fixed the trigger and the firing pin then used a lighter to burn the filament and then it became black and ugly, so a soldering iron is recommended.
I tend to use two screws to fix it on my phone but then I found that a double tape is enough, so the screw is just for the elastic now.
Use Cura to create the right part.

The fundamental is following https://www.thingiverse.com/thing:2960274, somehow my screen doesn't work with this, so I changed it to a non-contact structure.