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Most of the tube current is probably subcooling of the tube below the air temperature due to radiation to the cold night sky. If the tube is the same temperature as the air, there will be no convection current (other than from a possibly warm mirror, but i assume you have fans providing that cooling.
An easy, lighter and cheaper option than cork may be to stick space blanket radiative insulation to the outside of the tube. I did this to my big (550mm f4.6) dobsonian which I built with a 600 dia. galv. steel tube. I was getting a lot of tube currents and dew on the outside (which migrated to the cold mirror once back in the storage "cupboard" (long sort of garage I built for it in the real garage)).
Now that the outside is covered in gold bling space blanket, the tube does not feel cold and gets very little dew, and tube currents are much reduced.
Rebel sport sell the gold space blankets for about $6 for 210x160cm. either use double sided tape (I did) or spray contact adhesive to stick it on the outside of the tube. It's a little fragile but easily replaced. A couple of layers on top will be even better. Importantly, do not use sticky tape outside the space blanket, as the plastic sticky tape will have a high emissivity and will locally radiate and cool, defeating the purpose of the space blanket.
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