I'll start by assuming that you remove the vessel from the cooler when you want to raise the temp. If so I'd think our carboy heater should would suffice provided you wrap it and the FV with insulation. I generally use Fast Ferment FV's and use a carboy heater, wrapped with 5/8" closed cell foam cut from a camp sleep mat controlled by an Inkbird. I've built stands for the FF's and if I need to heat more than one vessel I stick a small milkhouse heater on the stand, under the FV and cover the lot with an appropriately sized cardboard box, also controlled by an Inkbird, it works very well. I also have a grow mat I've used in a pinch when I was fermenting in buckets, but it's ~3' long and 18" wide so most of the heat it generates is wasted, and it get a bit hot so the FV has to be raised a couple inches off the mat. As with most things related to brewing, there's a million ways to skin the cat, pick whatever works best for you and don't be afraid to try something that may seem crazy at first blush.