I've got one of those car windshield shades made of bubble insulation

Those aren't quite as good as bubble foil insulation, but they will work too. The bubbles aren't as big, and that's what traps your heat.

There are different types too. You can get single bubble, or double bubble. Double bubble is two layers of bubbles. There are also types with one White and one Foil layer on each side of the bubbles, and one with both sides made of Foil. I use single bubble with the White and put foil side down.

Double bubble works great, but it's bulky, and heavier, though neither weigh very much.

As per W-D's suggestion that you cinch up the draft collars, that does make a big difference. My bag has a cinch cord just above the shoulders, and another for the hood. I adjust them as needed, but always fumble around trying to get them loose.

"You want to go where?"