I dont have any experience since I live in NY and we're just starting to get to the warmer weather but I plan on using a bath towel or a thicker blanket and just drape it over the seat when LO isn't in it. I already keep those items in the car next to her seat so that's just easiest for me. I bet a crib sheet will do the same, I'd say if anything it may not be thick enough to block the sun (but could also depend on if you have tinted windows on your vehicle).
I just throw a blanket over the seat to make sure the buckles are covered. The fabric is usually fine anyway. I also drive 30 minutes after work to daycare so my car is typically cool before I get her.
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