I want to build a module that

Hello
I want to build a module that will sit in a large metal box. And the box wil bee filled with newspapers. When it gets full, I want a sensor detects that it is full. And it sends a mess to my phone or tablet, so I know that it is full.
Do you have something good ideas for this?
Greetings
Weija

There are various options.

You could have a load cell that weighs the papers sitting on it.

You could build a beam-break device (LED and photo-transistor) near the top of the box so when the pile gets that high it breaks the beam. You could ignore short term beam-breaks that are caused by newspapers being added to the box.

...R