How cold is it supposed to get? Your onions will probably be fine, mine have handled snow before. I think it's more cold that will get to them. Heck, I found an onion in a bed yesterday that had made it all winter here in CT! If you do make the covers post a pic, I'd love to see it.
& I do love those beds, a friend was supposed to make me one then COVID hit. I have plexiglass panels that fit my beds but they are a bit hard to move around. (I ripped my big toenail completely off last year getting one in place, lol)
So I cooked a pecan pie cheesecake in the pressure canner, looks good but won't eat it till tonight. I now have beef bones in there, started at an hour with no veggies. I will check on it & see if it needs longer to mush the bones & will decide if I want to add any veggies or herbs then. Then I'll can the broth. Then I'll do chicken. (still cleaning out the freezer here - look at me go!)
Some stupid squirrel tore through the bed with the walking onions.

I knew I should have covered it, my fault I suppose.