I got an MSM brand dew strip off Amazon (
https://www.amazon.com/Dew-Heater-fo...ef_=ast_sto_dp) that keeps my 72mm diagonal zoom lens nice and toasty. It runs off USB and has three power settings.
With the lens hood on the zoom lens I seem to only ever need to run it at the lowest power (in western Victoria, which is not necessarily known for being particularly humid ;-)
I can power it with a phone charging brick for a few hours if I use it with the DSLR or I can plug it into my asiair mini and run it off the battery pack that way.