Ving, What size is your finder? I made a heater for my 8 x 50 finder, using "Electric blanket" heater wire. I only did the objective lens cos when I'm not using it, I put the E/Pc cap back on.
The diam. of the 'big' end is 60 mm, so I used 2 turns,(approx 300 mm), of wire & tapped it about 60 mm in from the end to give me a high & low setting.
If you want more info. will try to draw diagram & post.
L.
Move post if nec. L.