I also have a WellMate composite tank and had no problem to wrap it in insulation. I also wrapped both the iron filter and softener. I used the foil and bubblewrap insulation. You can make cuts in it to form it to the curves and then use foil duct tape or vapor barrier Tuck tape.
I often wondered why someone doesn't come out with a wetsuit like material with a zipper.
BTW, I only wrapped the WellMate up to the high water mark. It doesn't sweat on the top third.