I use a very light no-name cotton cover. It is really only a dust cover and would be useless outside. However, that is not a problem since the snake lives indoors except when burning a little fuel.
I do have a couple friends who are old time collectors and they suggest only a light cotton cover at most preferrably nothing at all. A true outdoor cover keeps things dark which tend to be great homes for mice etc.
My cotton cover is literally soft and thin like a T-Shirt. Best of luck!