Buy one of this from Ebay:
Take a pair of 3D glasses (If you watch 3D films/movies you probably have a lot of them at home).
Take the measurement of the height of lens (in the middle of the frame).
It should be around 36mm.
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Step 1: Removing the 3D Lens From the Frame
It should take 3 sec. Just pop the lens out of the frame.
Step 2: Removing the Holder From the Loupe Lens
Even this operation should take no more than 3sec.
Step 3: Using a File Make 2 Notches Exactly in the Middle of the Frame
Take a rounded file and create 2 notches in the frame of the glasses.