I mocked up some cardboard over a Fisher - Price minivan. Will have to take some pics of it.
Sounds a bit silly, but hey, why not.
Basicially I came to the conclusion that putting it on and removing it, in cold weather, with gloves on, would have to be as quick as possible.
The top shell
, with magnets, would attach to hood, then has 2 side flaps and a front that flap down and hook to the bottom shell. Using bungies would snug those connections tight.
Top shell picture concept.
Yellow and blue areas are over hood. 2 sides drape down, so does a front bumper section:
The bottom shell
am thinking about would be aluminized tarp for the engine facing side, and abrasion resistant material for the ground facing side.
There would be 5 straps: one on each corner, and one strap in the center front.
Basically, you would push the bottom shell under the car from the front, so that the back corners are just under the exterior mirrors. The 2 back corner straps come up and attach to mirrors. The 2 front corners come up near the headlights and magnet to the hood. And one strap in the center brings the front of the shell up to cover the lower half of the bumper. It would magnet to hood.
Welding blanket tarps - bottom material facing engine:
This should snug it all together, and be portable and foldable. I think using a max of 1/2" to 1" extruded foam, cut in strips or sections, so that it can bend and fold like a gym mat, would be ample insulation.
Having the top go on after
the bottom means snow and rain would shed off and not get trapped in the sections of folds and seams.
I am not so sure it needs to "air tight" along the bottom. Having aluminized tarp material should reflect the heat up back into engine.
So anyway, all just theory, but I agree, a front clip insulated bag / cover / snuggie would keep it toasty.