Wanted a backplate for my Nest Thermostat to cover up the holes left from my previous older thermostat but I didnt like the large ones that Google sold that felt like they dwarfed the Nest.
My version covers any holes within a diameter of about 10cm around the Nest, which in my case was even just a bit big still but looks good enough.
Mates with the Nest Thermostat model number G4CVZ. Not sure if it would work with any others as their backplates are a bit different.
Ive included the f3d file below, so if you need a different diameter you should be able to modify it for your needs.
The author hasn't provided the model origin yet.