I used to have a plastic vari-kennel with a canvas cover over it in an open bed. When it got really cold, I would put an old sleeping bag over it. I'd come out in the mornings pull open the zipper and there would be steam coming out of the box and the dog was plenty cozy on a good fresh bed of hay.
If it were going to get really cold (like single digits) or was going to be really windy, I'd move the crate into the cab of the truck overnight. Dog was always comfortable. I think an insulated cover would do just as good as the crappy sleeping bag I had draped over my box.
If you have the dogs under a topper, I think a non-insulated canvas type of cover will be fine for the dogs with some hay. My buddy runs his dogs like that and has for some time and they seem warm even on those cold mornings.
My dogs are in a homemade wooden box under a topper with some hay today and it's in the single digits where I am at. Checked on them at lunch and they were just fine with one of the vents opened slightly on the box. I used to have a Garmin tempe sensor in there, but it must be broken (I swapped the battery and still can't pick up the signal), however, I remember that 2 dogs were able to keep the box at about 40 degrees above ambient. I've since put an insulated divider in the box (before was just a wood divider with air holes drilled in it) so that two dogs aren't heating the space for 4 and I am sure that helps when I have only two dogs in the box.
