This is a tool to replace the 13mm thick bottle cap recommended by Anycubic and position it in the middle of the bracket, makes life a bit easier when changing the FEP.
Size is 220x145mm
You'll need to make the top surface of the cylinder smooth to prevent scratching the FEP. Kapton tape for example works very well in my case.
The cover photo is from old version so it looks a bit different
The author hasn't provided the model origin yet.