I've been using a sunshield I made out of some black mat board for at least a decade now. There's a few little magnets in it to cling to the magnets hidden in my Mac's lid. It's been on like 3-5 computers now, looking increasingly battered; whenever I upgrade I get to see if the magnets are in the same orientation or not.

Works great, I keep it in the padded pocket in my laptop bag next to the computer and the drawing tablet and take it out when I'm sitting in the park.