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Markus, I had the same issue as you - it was hard/impossible to find a decent secondary dew heater for my 20" Skywatcher, so I ended up making my own for less than $15. I bought one of those USB powered heating pads off Ebay. If you strip the fabric apart, there's quite a decent length of plastic coated heating wire inside. I used silicone to spot-fix this wire evenly across the back of the secondary, then ran the cable along a spider vane, fixed with a long strip of black electrical tape. This makes a negligible difference to the thickness of the spider vane - certainly nothing that would make a perceivable diffraction spike issue at the eyepiece. The cable plugs into a small thumb-sized controller, the connection for which thankfully sits just outside the telescope frame. This then connects to your power source via the USB connection. Depending on the heating pad you buy, you may or may not need a usb extension cable to reach your power source. I've bought a couple of the heating pads (I also made a dew heater for my 6-slot filter slider), and they normally have a few different heat settings - even the lowest one seems to be sufficient to keep the dew away up here in Brisbane. I only run the heater and the GOTO off a Jayco Powertech battery, and while it drains a bit more power (the battery indicator level actually goes down one bar, rather than none at all), it still seems sufficient to get a couple of full nights out of it at least.
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