I used to hang mine Tempmate probe just in the air but the temp fluccuated too much
So I changed to strapping to the fermenter under some styrofoam, similar probs
Then I came up with strapping it to a Coopers PET bottle filled with water and insulated with camping mat, works well
Looks like a good setup. I've only just acquired an upright freezer capable of holding two fermenters. With one fermenter I simply bluetac the sensor to its side. However yesterday I put down two 23L brews plus a small 5L carboy of cider and didn't know what to do. Although all brews were put down on the same day, they all have different levels of activity 24hrs later due to different yeast pitched/volume ratios.
At the moment I've got the sensor hanging at mid fridge with a small quantity of bluetac around it to reduce temperature cycles.
How long have you used the extra water bottle trick. Does it accurately keep the air/fermentor temperatures under control?