How about 12vdc trace heating wrapped around said instruments get it to switch on under 20°C.
Or mini 20mm fans fitted to the cases to circulate the air through?
Desiccant packs inside and sealing the gaps are another simple option.
Or heated glass faces for the instruments?
Common for marine applications, it is a film that is applied to the glass.
Possibly a lot cheaper than MMC's heated glass, too.
More, here...
Hot Glass One problem, it needs 120v, but inverters are not expensive anymore...
