I got mines at walmart. But make sure you get the good thick ones, none of those thin foam flimsy stuff that don't work well.
If you want custom fit, you can buy a roll of foiled bubble wrap,
something like this, and trim it to fit.
I bought a roll myself to make shields for the side windows whenever I'm parked outside for more than a day.
Personal experience, it makes a huge difference in how much cooler it is compared to without them.
That pop top cover thing is nice, but I'd be paranoid of people, punk kids, stealing it. I also have a parking pass that needs to be visible, so I'd get ticketed with that on.