The sheet-style Rambo New market cooler is wonderful for your bottom cooler layer, but for the top you'll want a traditional square cooler that goes over the neck, too.
If you don't have to get your horse wet when the temperatures dip, great! Eventually, even heavy mud will dry and be curried off. Sometimes, though, there's no choice and you've got to give the horse a bath.
As long as the horse is healthy - meaning he's not very old, very young, prone to respiratory or immune problems or has an illness or diarrhea - you should be able to safely bathe your horse in the winter. Ideally, you'll have a draft-free wash stall with warm water. Of course, most barns are far from ideal . . .
Try to bathe the horse inside, away from drafts, even if it means in his stall where the bedding can soak up water (so you don't get icy patches) and then be replaced. Use buckets and a sponge instead of a hose,
and wash only the areas you must. For example, if he needs a medicated-shampoo bath because of a breakout on his rump, just bathe the butt.
Waterless, aka "dry," shampoos are also an excellent choice for cold-weather cleanups, especially if it's a limited area, such as socks.
If you must bathe the entire horse, do the front half first, then the back half, putting a cooler over the wet area to keep him warm while you work the second area.
Use lukewarm water for both the bath and the rinse, and avoid shampoos that must sit on the horse for so many minutes before rinsing.
Note: We won't use dishwashing liquid. As much as people think it's just dandy for bathing a horse, it will leave the skin and coat dry, which is particularly troublesome in the winter when the air is already causing dryness. And dry, itchy skin opens pathways for infectious skin disease, and dirty blankets can also lead to skin problems.
As you bathe, scrape off excess water as quickly as possible, then rub the wet area briskly with a thick terrycloth towel (you'll want to have several of these available for the bath).
After towel drying the horse, place two clean fleece coolers on the horse and leave them there until the horse is completely dry and warm. You can use a sheet-type cooler for the underneath layer, but a large, square traditional cooler is best for the top layer because it will cover him ears to tail. Tie or clip the front closed.
Old timers used to place a layer of fluffed-up straw under the coolers. This allowed the moisture to rise up into the top cooler as it evaporated, keeping it off of the horse's back. It's still a great idea, if you have straw available. Either way, replace the two coolers when they become damp with two dry coolers. Leaving a damp cooler on your horse can cause him to become too chilled.
If your horse has a thick coat, it'll take awhile for him to dry. Giving a horse a bath is different from him getting wet in turnout. Chances are his hair kept the snow or a light rain off of his skin, but a bath gets right down to the skin. (Note: We'd avoid the use of heat lamps in a barn.)