You know those windshield reflectors? I got a huge one on clearance at an auto parts store, was originally $17 - I got it for $6. I swear, the thing was made for a Hummer or something. Anyway, I cut it up and taped it to the sides of my tank (on the outside, obviously) then put a blanket on top. Works like a charm, if you get a warm blanket (such as flannel) especially so.