I've seen them on doors:

1. Large letter flap with a large slot behind it so it will accept
telephone directories, small packets etc. rather than just letters
2. Inward opening
3. Weak spring so even lightweight post will drop and not get caught

but cannot find a supplier who sells them. Anyone know where I can buy
such a thing?

On the DIY theme are there any issues with moving the letter slot from
one part of a front door to another? For example, planning permission.
I'd like to move it from the bottom of the door to the middle so I can
catch the mail behind the door. Of course, I'd need a way to block up
the existing slot somehow.

Any help appreciated.

--
James