AETHERPHON, global theremin family

There are a couple of other options that come to mind.

Assuming you feel your home is that vulnerable, you might consider a 
small, climate-controlled storage unit.  Even if you have to pay for a 
whole month, you can store your most valuable possesions temoparily for 
a fraction of the price of insurance or replacement.  That's assuming 
the construction of the storage units is superior to your dwelling.

Another option, if you have the time, is to build a very strong crate 
to house your theremin, in addition to the interior closet.  You could 
not only wrap the theremin very tightly in plastic, the crate could be 
painted on the outside with a water-proof epoxy to thwart the moisture 
even further.  If you're smart, you can build the crate so it can be 
broken down for storage when not in use.

If you are going to seal your theremin in plastic, I would suggest 
getting one of those silica gel desiccant packets to put inside before 
you seal it up.