Do a wire basket with wires spaced pretty far apart. That would catch the letters but not bugs.

" the problem is that insects like cockroaches crawl inside. "

Going to be close to impossible. Is there is even a miniscule opening they will find a way in.

Thank you.

I didn't quite understand what kind of wire basket I can make, Google images gives me these kind of results. This of course does not prevent bugs.

What does your Landlord say?

Inhave not ask them about this. But I believe they will allow me to change things.

If it is not correct, the landlord is responsible to repair!

You can buy letter catchers or letter cages. They are widely used here where letters go straight through the front door into the house and some people have chewy dogs. Like this: Buy Argos Home Wall Mountable Letter Cage - Chrome Finish | Door furniture | Argos You'd empty it every day so I don't see why bugs would be a problem.

That is good to catch letters. The issue is that bugs crawl from outside because the front part does not close well. I did not explain this properly... It leaves a gap where I marked in red in picture. Perhaps I need something to hold it shut, I'll try some thing that I can find....

Thanks