Here's one I've learned from a lot of observation & using different things to cover FV's with. Fleece lined CPO's & other insulated coats will make the temp go up a little more than other materials. Dark tee shirts are Good for keeping light out,but are more suceptable to ambient air changes. My old black velvet smoking jacket actually helped maintain temps the best. I have two batches going atm,brewed in the same week. Both with the same yeast. The dark lager is in the Cooper's micro brew FV with the black velvet covering it. It started at 64F,& never went above about 69F. At 68F as of yesterday about 4:30pm. The other with a light colored ale was covered with a dark tee shirt,& started at 64F,at 22C (71.6F) as of the same time yesterday. Both are past initial fermentation by a few days.
I'm thinking of having the wife help me figure out how many yards of black velvet it'll take to make covers for both FV's to fit. Get the temp right inside the fermenter,& the black velvet seems to maintain it better with less interferance from ambient temps.