I've been looking at the amazing range of prices for what is essentially the same item...
A well known online bee supplies shop sell a varrox vaporiser - £140
I bought a "Aluminium enclosed 12V 80W PTC Heater/Heating Plate ~175℃ Max Temperature" from a popular online auction site - £7
What's the difference? Well, the expensive item has a nice label on it, and doesn't come supplied with a long lead and battery clips (I had some in the shed, but could have bought new for around a fiver)
I just screwed the PTC plate to a small piece of scrap plywood to stabilise it (the cable I used, however I lay it out, likes to keep twisting) I tested mine outside in the cold this afternoon, and it worked a treat, so now I'm all set up for treating them after christmas.
I have a digital thermometer on a multi-meter, so watched the temperature of the PTC plate during the test.
The plate was inside an empty super (to protect it from the cold breeze, and replicate it being in a hive)
It took 3 minutes to get up to 178C, and then held that temperature +/-2C for the 4 minutes I had the battery connected.