I had the same problem once on my small FS60. A few plain old rubber bands around the lens cell and tube did the trick.
You could also try and heat the dew shield near the lens cell (using a dew heating strap or something) to increase its diameter slightly - after letting the scope cool down for a while outside
and making sure the lens cell stays as cool as possible relative to the dew shield.
Otherwise I'm afraid you
will have to sell it.
