I wouldn't worry too much about covering them. Just be sure the top covers are secure so they don't blow off.
If they are in an exposed location, perhaps set up some kind of wind block for them, if possible (ie: haybales or cord wood piles).
From my experience, most of my colony winter die-offs have been due to damp/wet/moldy hives (poor air circulation = condensation).
Keep an eye open for moisture build-up on underside of top cover. Maybe look into making or getting a "quilt box" for the top if you start to notice any condensation.
Another method is to lay a few popsicle sticks between the inner cover and top cover to add a bit of air flow.
A local bee keeper also recommended treating bees for varroa in the Fall to increase their odds getting through winter.
I've used Apivar for this before, it's good for when the honey supers are off and the bees no longer have capped brood.
I personally prefer not to use chemical treatments if I can get by without them, but I also figure a little preventative care is better than a dead hive.
"He that plants trees loves others beside himself" -Thomas Fuller
I live next to three of the areas biggest farmers. I am kidding myself if I think my honey is organic.
Thanks for the advice. The bees sit in a small orchard that is surrounded by woods. Wind is not a concern. A concrete block is on top of the hive.
I raised bees before, but that was in northern MN. There the temps dropped to 50 below and 40 below was common. Here I saw 18 below maybe once and 0, or a tad below, is common. So, the winter needs are different. I will probably make a box to temporarily drop over the hive if the temp spikes low.
Good decisions come from experience. Experience comes from making bad decisions. Mark Twain