Assuming it's an unheated garage I'd go for the same level of insulation as even though it's out of the wind it's still going to be a similar temperature in there as outside. Some people use perspex or similar on the front of the hutch so it's not constantly dark but still offers some insulation during the day and for general covers you can use carpet, old duvets, polystyrene/foil insulation and of course hutch huggers. The tube heaters would probably need a guard but I know some people have used them in hutches and I've seen at least one site selling them and recommending them for that use but I can't vouch for how safe they are. I know they rely on air movement to dissipate the heat so I don't know if it would overheat in a covered hutch (although you can get thermostatically controlled ones and they have a thermal cut out). Another option would be a heatpad such as the snuggle safe (microwaveable) or an electric heatpad such as the petnap flexi-guard one.
