If you're really worried about getting your Zilla too hot, get an EGT.  By the time your water temp is skyrocketing, your top-end could already be toast.  An EGT won't prevent a meltdown either, but it will feed you a more accurate response 20x faster than the water temp will.

This is about as cheap as you can find, but you will need a power source. 

About the best way to make sure you aren't overheating is to properly jet your bike with a plug chop and do a leakdown test.  Air leaks and wrong jetting are probably the biggest top-end killers out there.