A bit more adaptable than the fermenter tray, especially if you ferment lots of single gallons.
Ok for glass or plastic demi-johns!
If you put a stick on thermometer on one of the demi johns you'll know what temperature they are at a glance!
You can put an old jumper over the demijohns to raise the temperature a bit, make sure you let some air through or it soon gets too hot! We wouldn't recommend covering the heating tray!
You may need to experiment a bit to get the temperature where you prefer it, mostly they're ok just as they are!