Trevor,
Some time ago I posted some notes on home-build right angle viewfinder, see this:
http://www.iceinspace.com.au/forum/s...ad.php?t=23756
It may help...
However, I found that I can fix the position of the focus ring once the focus is determined, as camera does go back in the same position every time (unless something else is moved in the meantime). so there is no need for me to re-focus every time again and again..
Also the thread on Bahtinov's mask may be helpful for you:
http://www.iceinspace.com.au/forum/s...ad.php?t=35676