coverhulls work pretty well for me, but they too will crust over if you have some hard frost or heavy moisture periods. dry dirt mixed with calcium or even table salt works ok too, but it will freeze if it draws out enough moisture from the air, rain, or the surrounding soils.
here are couple of things I do in freeze/thaw conditions:
1.) line the trap bed with a plastic baggy, (sandwich bag, sections cut from black trash bags, etc.) then pour in your dry salted dirt, bed your trap in that, then sift a little more over everything. The plastic trick works when the surround soil is damp and will keep your dry dirt from pulling that moisture into your trap bed.
2.) if the ground has moisture in it, dig out your trap bed and simply push the trap frame down into the moist soil. if you are using coil spring traps, be sure to not let the jaws, levers, or spring coils touch the wet soil in the bottom or sides of the bed. Then fill the rest of the trap bed with dry dirt or coverhulls. In freezing conditions, that trap frame will freeze down a little making the trap rock-solid in the bed while the "working" parts are in a non-moisture material and can fully function. Coverhulls work best for me in this scenario.
3.) where you put your sets will also be a factor. if the average temps are around freezing point both day and night, put your sets in the "shade" so that direct sunlight on the set will never let things thaw out enough to be an issue. Example, make a set on the backside of a haybale, fencerow, just inside a thick grove of trees, etc. If the sunlight can't hit the set and especially the bedding & covering material, it will be much less likely to thaw out during the day and then freeze hard that night. If it is constantly "frozen" in the shade, then dry dirt or whatever covering material you use will always be dry and work...at least until a few nights of frost, light rain, snow, etc. apply too much moisture to the location.
Only other advice I can give from my own experience is that it is just plain a lot of work, but you do get back out of it about what you put into it also. If I neglect to remake any sets that are "iffy" as to whether they will fire or not that night, then I take a huge chance of missing animals. If I am not 100% confident on the sets performance that night, I rebed the trap with new dry or freeze-proof material. It is always worth the extra effort, in my opinion. I always put up way more dry dirt than I actually use, too (just in case things get wet or the freeze/thaw game takes more material in remakes than I planned on).
good luck and have fun!
Jim-NE
