How about this?
http://www.craftsman.com/shc/s/p_101...4&blockType=L4
It'll last forever as long as you treat it as a precision measuring tool and not use it for general wrenching. It'll never go out of calibration because it depends upon the simple principal of bending the handle.
A 3/8 socket of the right size for your cassette tool is all you need to "adapt it".
Shimano cassettes come with a lockring.