GaryS

Let's put it this way, your setup doesn't allow for air circulation and that may be the limiting factor, top on or off.

I used to overwinter my stuff when I lived in an apartment in Chicago 30 years ago.

I built a cold frame out of wood and put it on the balcony. Inside I had pipe heating coils that I bought at the hardware store. I had an outside outlet and I would plug it in when it got really cold. A little mulch under and to the top of the pots. A minimum/maximum thermometer inside to keep track of the temps in the box.

It worked.

Seems you could do the same with your big can functioning as a cold frame.
Take the top off during warm spells and put it on at night and during cold spells.

23ºF is the coldest you want it to get for Trident maples. At least that's my experience. 28º is ideal. They stay frozen but the roots don't reach killing temps.

Good luck!

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GaryS