Letterbox replacement with an odd shaped aperture.

[Andrea]

Hello. I removed the broken letterbox from my UPVC front door and am unsure how to proceed. The aperture is a strange shape. At it's widest the hole is 250mm and the top and bottom edges are 218mm. I bought a 10inch letterbox as the old one was about that but it was slightly too small and didn't fit. Has anyone seen a hole like this before? If so do you know what size letterbox I should go for? I imagine a 12 inch would be too big. Will the hole need resizing?
Thank you

 

[Andrea]

10inch looked about right but the new letterbox is a few mm shorter than the old one (and the telescopic bit inside is about an inch shorter than the old one - the new one is at the top of the picture)