Letterbox / letter flap for sliding porch door

Hotblack Desiato

We have a sliding porch door which had two plastic coated steel letter flaps one on each side which were originally spring loaded, the springs departed quite a while ago, the inside flap has now broken off entirely and the outer one looks like it might soon do the same.

The door is made out of aluminium and the thickness where the letter opening is is about 5mm. Problem is, in store and online I can only find letterboxes (with two sides) which fit wooden or composite doors of 25mm thickness or more.

We'd like a similar arrangement to what we have now - a hinged flap on each side of the door. I've tried B&Q, Screwfix, Woodies, Amazon, eBay, Brooks, and a few builders providers with no luck. Any ideas?

Hotblack Desiato

Also the original two flaps are pop riveted to each other, I have a pop riveter but the fixing holes would need to be in the same places, is this standard?

There are quite a few double letter flap arrangements which have a plastic 'tunnel' in between them which could, I suppose, be cut down to allow a thinner door thickness, but I suspect these are designed to be secured to the door with woodscrews, none of them online show how they're actually secured to the door.

Hotblack Desiato

I can't be the only person on Boards with a sliding porch door and a letterbox, can I? 😟