We've got a 24' yurt from
and had been suffering from the immense heat created by sun through the dome during the summer months here. To make things trickier, our yurt has a ceiling fan, so a lot of the solutions I've seen elsewhere wouldn't fit our dome.
What finally worked? A collapsible photography reflector. It's made of a highly reflective silver fabric on one side and a white fabric on the other. The outside is a band of spring steel that provides outward tension. It's super lightweight, cost $40, and is available on Amazon:
https://amzn.to/3RpYDIT
I kinda nestled it into place and its own tension holds itself securely in the crown. It doesn't even interfere with opening and closing the dome, but it does a tremendous job of reflecting direct sunlight back out of the yurt.
Hope this approach is helpful to others!