It is stable to hold the mobile phone because I use an extended leg. The leg is also adjustable. 
To withstand the weight of the smartphone, I designed the supporting leg so that it can be customized. If you want to tilt forward, then the weight of the phone holder is in front. We can move the footstool to the front so that the phone holder is more stable to withstand the load on the front.
vice versa, if you want to tilt backward, you only need to move the supporting leg to the back. by default, the support leg can be installed in the middle. And it's stable enough to be a phone holder on a desk.
- need improvement:
Cut unnecessary parts to reduce material when printed. Anyone interested in upgrading this project can see the link of tinkercad