The Medway Handyman wrote:

Thus saving the trouble of cutting a letterbox size slot in a door - and
fitting one of the **** poor designed letterboxes. Why they can't recess
the spring is beyond me, cutting out a slot to take it is a right PITA.

I've had a couple now, of the type where both outside and inside
portions are the same, with sprung and sealed flaps, but with one part
sliding inside the other. They function well and are completely draft
proof, apart from when the paper guy doesn't push it right through !

Andy C