You can get sheets of insulation cheap from home depot/lowes for this purpose. I did so with an open air TGR rack in my quarantine room/closet. Can’t really say how well it worked because I didn’t do a before/after test. I’ve since got a little mini oil filled heater to keep the ambients up and dont need the insulation.
You might be surprised how well a $75 oil filled heater can do to get those ambient temps up. If you’re going to go the professional freedom breeder rack level I’d be looking into effective ways to heat the room. You can likely insulate one enough to get by similarly to the enclosed racks you are used to but I wouldn’t think of it as a permanent solution.