Window covers do three separate jobs: they block sightlines into the car, they slow heat moving through the glass, and they keep parking lot lighting off your face. No single material does all three well, so pick by which job you actually need tonight and expect to own two kinds.
Start with what you are covering for
Privacy is usually the reason people buy covers, and it is worth being precise about it. Covers stop a casual look through the glass. They do not make the car unoccupied, they do not hide the car, and in the wrong setting they announce it. More on that below, because it is the part most guides skip.
Heat is the second job and the more measurable one. Glass is the thinnest, least insulating surface in the vehicle. In summer sun it lets radiant heat straight in; on a clear cold night it dumps heat straight out and drops to near outside temperature, which is why it becomes the wettest surface in the car.
Light is the third and the easiest. Any opaque panel solves it.
Reflective foam, fabric, and cardboard
Foil-faced foam or bubble panel is the windshield-sunshade material sold by the roll. It works mainly by reflecting radiant heat rather than by resisting conduction, so the shiny face wants a small air space in front of it rather than being pressed flat against the glass. It is light, it cuts fast with a utility knife, it holds a friction fit well, and it is opaque. Its weakness is appearance: silver panels in every window are visible from a block away at night.
Blackout fabric, either sewn into panels with a stiffener or hung on a rod or wire, is the best at light and by far the most normal-looking from outside, especially in a dark color. Thermally it does very little beyond breaking the draft off the glass. It packs down small, which matters in a car where storage is the whole fight.
Cardboard cut from a box is genuinely fine as a prototype and as a one-trip solution. It warps the first time it gets damp, which in a car that runs with condensation is the first night.
Reflectix-style panel with a fabric face is the common compromise: foam for the thermal job, dark cloth on the outward side so the car reads as ordinary. You can make this with spray adhesive in an afternoon.
Attachment, in order of reliability
Friction fit is the most reliable and needs no hardware. Template the pane in cardboard or paper, transfer the shape, then cut the final panel about a quarter inch oversize on every edge. The foam compresses into the window reveal and stays. Test-fit before you cut all of them, because a panel cut for the left rear will not fit the right rear on most cars.
Magnets work well and only where the frame is steel. Check every window with a refrigerator magnet before buying a magnetic kit, on the exact contact point you plan to use. Plastic trim around the glass is common and it is invisible until you test.
Tucking into the rubber seal works on the fixed rear quarter glass of many vehicles and nowhere else. Costs nothing to try.
Suction cups are the worst of the options. They release in cold, which is exactly when you want the cover up, and they leave rings on the glass. Use them for the windshield if at all.
The condensation trap
A cover held tight against cold glass traps humid air against the coldest surface in the car. The National Weather Service explains why that surface matters: the dew point is the temperature air has to be cooled to in order to reach 100 percent relative humidity, and cooling past it forces water vapor out as liquid. Your covered glass is still the thing hitting that temperature first, and now there is a panel holding moist air against it.
So covers are not a moisture fix. Two things help. Leave the panel slightly proud of the glass rather than sealed to it where the fit allows, so the trapped layer is not pinned. And keep two windows cracked on opposite sides regardless of what the covers are doing, which is the actual condensation answer and is covered in how to stop condensation in a car. Cut a notch in the top of the panels at the two windows you plan to crack, so putting the covers up does not quietly seal the car.
The unhelpful part: covered windows are conspicuous
A car with every window blacked out reads as occupied. That is the trade, and where you are decides whether it costs you anything.
In a rest area or truck stop full of vehicles with sunshades and sleeper curtains, nobody registers it. On a residential street, silver panels in the side windows are the most visible signal a car can give, and they are what a neighbor calls about. A dark fabric cover or a plain windshield sunshade with the side glass left alone attracts far less attention than a full silver kit.
Two more honest costs. Covers block your view out, so you cannot see who is next to the car without moving one, which is a real trade in a lot you are not sure about: rest area safety covers how to weigh that. And a full set takes real time to put up and take down, which means on a marginal night you will be tempted to skip it and sleep with the glass bare.
Windshield and the front seats
The windshield is the biggest single pane and the one people see through from the most angles. A standard folding sunshade is normal on any parked car in daylight and reads as nothing at all. At night, a dark cloth or towel over the sunshade adds the light and privacy that reflective material alone does not. Sun visors down plus a cover across the front seat headrests closes off the last sightline through the front glass.
Take the windshield shade down before you drive. Never drive with side or rear covers up.
Making a set in an afternoon
Template every pane with paper or cardboard taped in place, then trace onto the panel material. Cut a quarter inch oversize with a fresh utility blade against a straightedge. Label the back of each panel with its window (LR, RR, hatch) in marker, because you will be sorting them in the dark. Store the set flat under the sleeping platform or in a slim pocket behind a seat: see building a sleeping platform for where that space comes from.
Covers change how visible you are. They do not change whether you are allowed to be there. That is a local question with a local answer: check where it is legal to sleep in your car, and remember the posted sign and whoever manages the lot beat anything on this site.