While attempting to separate the heater wire from an electric blanket (ever tried to do that? What a mission !! ) I had a brainwave.
I used to do homebrew and in the cooler months used to put a small heater pad under the brew vessel. I just dug it out, 6 watts in a flexible pad form with power connections all ready to go !
I am just going to make up an insulating sleeve with Velcro or similar and wrap that against the underside of the OTA just below the Dew shield.
I might even have a ready made controller for it ....