Safe, cheap, heated enclosure solution for New England garage
The easiest step would be to add some isolation on the outside of the box. If you have space inside, there might be a good spot to store some non-flammable insulation, for example rock or glassfiber wool.
A different material might also be possible - firebrick is not only non-flammable but also a very good insulator! about 2-3 inches of firebrick can contain the heat produced by a tempering oven while the outside is cool enough to be safe to touch with gloves.
However, you should install an extra thermosensor for the chamber temperature - and make sure that the printer enters print halt mode once the temperature in the chamber gets above a critical temperature to try to mitigate fire risks and possibly start a chamber-cooling protocol - which might include aborting the print or activating coolers that rapidly cool down the chamber.
As you pretty much are going heated chamber, you might find a spot when you might want to use a flexible hose to pull non-chamber air to supply the parts cooling fan.
Also, possibly relocate as much of the electronics to a compartment separate from the printer's heat chamber to ensure the electronics don't get cooked and can be supplied with cooler air.