On my Skywatcher 120mm Achromat (and every other telescope I own), I use a 12 Volt hair dryer that I plug into into a car battery starter.
It costs about $24 on Ebay.
https://www.ebay.com.au/itm/12V-Hair...0AAOSwX1xc3nC9
On nights when everything gets covered with dew, I just blow the warm air on the objective lens, finderscope, red dot finder and eyepiece. This is usually good for another 15 to 20 minutes of viewing.
Only problem one can have is that the hair dryer might not work when plugged into the cigarette lighter sockets of various power units, some of which cut out when that much power is drawn from them. This doesn't happen with my Projecta car battery starter, but did happen with a cheap battery starter unit from Aldi. Solution there was to buy a 12 Volt socket extension cord, cut the base off, separate the wires, strip the two ends, and attach them to the jump start cables (or even cheaper, cut the 12 volt plug off the hair dryer, and attach its wires to the jump start cables).
To stop the refractor and finderscope from dewing over as quickly, and to extend the time between having to blow dew off, many years ago there was an article in Sky and Telescope showing how attaching aluminium foil to dew shields stopped them dewing up as quickly. So I made dewshields out of plastic from cheap folders for my finderscopes, then attached aluminium foil to the rolled up little dewshields, covered my Telrad units and their dewshields with that foil using Blutack, and also used BluTack to stick aluminium foil to my telescopes' flexible dewshields. This worked and I though it a worthwhile exercise back in the days when I did heaps of viewing in all night sessions. But as a mainly intermittent viewer nowadays, the 12 volt hair dryer is good enough for me.
Regards,
Renato