I think you're missing the most obvious solution. Why are you sticking your volume measuring device into the wort? Why not measure from a fixed point on the top of the tun? You still calibrate it the same way, but you don't have to immerse anything.
+1, I had a metal yardstick that no longer has all the markings...now I measure from the top and plug into a spreadsheet to do the hard math. However, I recently upgraded to a Blichmann 20-gallon boil kettle and now REALLY want site gages on my other two vessels...looking the the MB site gages for my 26-gallon HLT and 15-gallon MT.