I had one of the those staywell ones for years in London, simple and effective.
Cant imagine why it has bed reviews.
I have a fourway locking flap that I put into a pvc double glazed door here, not as easy to pur in as in a wood door,,,,,,,,,,or is it I am just older and doing things at low level with a balance problem is hard!
I cant remember if the staywell one comes with the fixings for a thicker door so you would have to check.