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Not the air I'm talking about keeping hot, it's 200kg of engine, steel, alloy & fluids etc.how many kilos of salt and plumbing will you need to match that thermal mass?
Insulate & seal the bonnet, stop the heat venting out the top, the firewall is usually pretty well insulated to the cabin anyway, insulate the side guards, the bottom doesn't so much need to be insulated as properly sealed.
When driving the rear bottom vent is opened to let air out, and the normal radiator opening is left open to let air in, as much as required by the conditions, when parked, close lower vent and front grille area, no need for additional fans.
Once you stop convection, the heat stays in, and any added insulation minimises the conductive losses.
I think if you pull out some numbers on it you will find you will need to fill the entire trunk with atleast 50kg of salt, with all the associated equipment, just to get engine from ambient cold to 40C.
Remember the salt can't give up all it's heat, it can only equalise the engine temp.
Let's say you have 50kg salt at 100c and it stores twice the heat of steel etc.
You have 200kg of engine at 10C, best you are going to get is 55C across everything, but remember as the two bodies get closer in temp, the rate of heat transfer slows down, so you will get it to 40C fairly quickly, but to get that last 15C might take half an hour of pumping coolant through.
If you can keep the engine hot enough, say 40C by the end of a working day, then you just, open your vents, get in and drive away.
If I had to deal with the extreme cold issues, that's what I would look at first.
Edit: Just thought, has anyone just tried using one of those cheap car covers to see what difference in heat loss from engine bay area is, just trapping a layer of air may actually be quite effective for a few hours?
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