I built my own dew heater ring and controller for my Celestron C8 very simply using nichrome wire wrapped in duct tape and a cheap PWM controller from Amazon. The PWM is nice because it's more efficient than controlling current in other ways so somewhat offsets the loss off any battery usage for using a higher temp than necessary.
I followed the instructions at
https://www.deepskywatch.com/Article...ew-heater.html for the heater strap. I ran two wires in parallel to fir the internal circumference of the OTA, I then connected this to a 5.5/2.1mm DC plug. I just taped it on the inside of the OTA making sure I left enough of a gap at the front to be able to use the cover. I added velcro to the cover and the end of the plug so that I could attach it to the cover and reduce the amount of contact the wiring might make with the optics when the cover is closed.
I mounted the PWM inside a small enclosure, and wired it up with 2 5.5/2.1mm DC sockets and then attached it to my dew shield next to a small hole through which the plug can run.
I played with the temperature a little bit and have found a 'max' value where there is slight star distortion due to the heat. That value will depend on the input amps.
All up it cost me approx $30 and most time was spent running the fiddly nichrome wire along the duct tape so that I had a consistent, straight ring.
Nichrome:
https://www.jaycar.com.au/nichrome-r...-pack/p/WW4040
PWM I used:
https://www.amazon.com.au/Controller...d=AXQ2EGXUO12M