Finished cover stuck on the fridge
Insulating fabric from Joann's. It's called "Warm Window". It has two layers of reflective material sandwiching three layers of thin batting which creates the airspace you need.
I then covered that with simple black fabric for the exterior and a decorative fabric for the interior. Black makes the windows look tinted and not like there are insulated window covers in the window - which to us would look super obvious that it's a camper. This way it looks way more stealth.
Don't worry! The black fabric layer doesn't hinder the reflective properties.
Full disclosure: I have some sewing skills and access to a nice machine.
After measuring the windows and comparing that to the size of the "warm window" fabric dimensions (which can be cut to whatever length you want), I made "pillow cases" sort of for the insulating fabric. In these cases, I sewed pockets in for the rare earth magnets. This is only going to in for the back windows. The side windows don't need magnets because the window frames are aluminum. I used velcro on those. Unless you want to make the window covers larger than your frames so they can stick to the frame of the van. But also take into account how you'll frame the windows. My framing/trim came all the way up the aluminum frame.
I should have added more magnets. They hold up fine, but I rather have them really grab on good! I spaced them about 7-9 inches apart. I will be adding magnets in between them all, so they will be spaced 3-4 inches apart.
They work amazingly well!! We've woken up when the sun was totally out, and boy were they hot on the window side but we were nice and cool inside. Vice versa for the cold nights. You can feel behind them and it's definitely keeping the heat in or out whatever you need.
So happy I went with black for the exterior.