Here is a commercial version using it's own name "Sky Window".
http://www.tricomachine.com/skywindow/
Expensive, apparently - up to USD300 for mirror and mount.
Perhaps get an old photocopier mirror - I think they are front surface coated and should be cheap if not free?
OK a bit of searching leads to - try searching "binocular mirror mount"
The first hit has a set of links at the bottom:-
http://www.craigcolvin.com/Astronomy...larMirror.html