Re: Sticky back Foil.
Does it have to be mirrored?
What about 'stained glass'?
Peels of London do rolls of stained glass effect translucent plastic films that you roll onto a clean window with soapy water.
It stays put by surface tension, I suppose, but you can peel it off if you want to. No glue or sticky stuff involved.
I've got some on two windows that have a bad view, and as the light shines through, it is just like stained glass! It's been there about 15 years now.
All sorts of designs.
Here's the link......
http://www.e-peels.co.uk