I use a lot of cover hulls on my dryland sets. As Asa mentioned though, some of my success may come from fact I trap mainly farmland here in midwest.
Coverhulls will freeze if you get a rain then a hard freeze. They will also "crust over" after several nights of freeze/thaw conditions, as we seem to experience around here.
I try to make my sets in the shade now. Seems to go against the grain a bit, but I've found if I keep my dry sets in the colder areas on location, like the shady side of a hay bale, or fencerow, or overgrown thicket, etc. that the sun never hits them and they don't seem to fall prey to freeze/thaw as readily as my sun-exposed sets do. If they are going to freeze anyway overnight, keep the sets in an area where the immediate ground surrounding the trap bed stays cold. Seems to help, anyway.
I also use a wad of insulation under my trap pan, too. I dig the trap bed, line the trap bed with a couple of sandwich baggies, or piece of black plastic garbage bag, etc. then press the set trap with insulation under the pan right down into the bed. The plastic keeps ground moisture from seeping back into the cover hulls or working parts of the trap. Then I pour a generous layer of cover hulls on the trap much like any other cover material, smooth out a place over the pan and rest of general target area. If i want to blend in the set a little more, I might chop up some dead grasses very finely and sprinkle them over the cover hulls. If wind is a problem blowing the hulls off of the trap, a light layer of dry dirt works, too.
