Submitted by Eric Daniel
This may seem a little morbid, it certainly did to more than a couple of folk in my unit, but back in the early 90's the best way we found to keep the wind and weather out and the heat in while sleeping on the tank was to get yourself a body bag.
Officially designated a "human remains pouch, type I" the bag I used, which I "acquired" during Desert Storm, is a zippered nylon sack and, by design, is big enough to accommodate not only me, but my sleeping bag, and my sensitive items.
While the nylon fabric is not totally waterproof, it is fairly resistant, and when treated with Scotchguard (or its ozone-friendly 3M equivalent) holds up well to a pretty thorough soaking. Moreover, it does a very good job of cutting down on any crosswind and has even proven itself capable of seeing me through near zero snowstorms while out in Yakima. Finally, it is also pretty durable; I'm still using the same bag I picked up back in 91.
Granted, like just about everything else I use or have commented on, there are bigger and better items out there. The Army itself has putout a gortex bivy sack to go with its new modular sleeping bag (I'm on my second M1949 mountain bag) which is definitely the cat's meow, but at $275 retail (compared to $15 for the body bag) the Army bag is a little on the spendy side.