you could lower it as little as neccessary and put foil backed foam insualtion around it.
Probably the best insualtor for the thickness of it, which starts at 25mm and goes up to 150mm. Don't forget to do a lid for it and cut a hole for the expansion pipe to sit over the tank. If you can reach the rising main pipe (usually black plastic pipe) you should wrap it with rockwool also.
Watch the use of heaters though. Dangerous things in an attic. If the temperature in the attic is a big worry, get an electric thermometer and wire it in. I have one that sounds a buzzer if the temperature drops to a level I determined - 5 dcel.
Ebay have them. Mini digital LCD temperature is the search string.