Hmmm... I can see you are trying different things and be innovative, but have you considered just leaving them outside until the temperature fall to the point of constant hard freeze? Almost any northern deciduous trees and even seedlings are Absolutely OK until mid-20’s once temps fall below to teens things get a little dicey, and single digits can be harmful. So what you really need to do is find a place outside or in sheltered area of your home that they can be kept protected.
Small trees can be harmed if the roots freeze hard, so it’s a good idea to wrap the roots in their containers and base of the tree. Putting in an insulated container (with drain holes) like old picnic cooler or doubled wooden crate stuffed with insulating material is not a bad idea. But you could just as well bury the pot, cover the tree with hardware cloth (1/2” — 1” would let mice, etc. rodents in) and stuff with dry leaves or straw. All of this is after they have dropped leaves and gone dormant.
Bonsai are often kept in coldframes or under the porch or the house, north side of the house, etc. Covered with leaves and tarp. Apparently, sunny south side of the house which seems like a good idea because it’s warmer is actually not suitable because there is more thaw/freeze cycle (I didn’t know that until a few years ago so I thought I would mention it).
Around late November, before the ground freezes completely, I move in the ones I really don’t trust to the outside mid-winter elements to my unheated garage, where depth of winter temperature falls to low 20’s, killing some of my more sensitive plants, but trees are usually OK. If I really want them to stay above mid-20’s I move them to the insulated side of the garage near the laundry room door, and sometimes cover them on worst nights. I do have a thermometer in the garage, but I also keep buckets and containers of water nearby the plants as a crude temperature indicator — you know, thin ice, thick ice, frozen solid, etc.
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