I used to put reflectix up there in storage, but it was hard to get it to stay.
When my roof was done by rvroof.com a few years ago they gave me the option to coat the skylight. I replaced it with new since the plastic was badly UV damaged, and then had them coat it. Makes the sower dark but significantly cooled down the joint and eliminated the UV damage to the interior finishes. I'm very glad I did it. The other upshot is tat it made the skylight a lot tougher against things such as hail or unidentified falling objects.
If not doing rvroof.com, I would at least paint the dome. I like the flexseal idea but that probably wouldn't hold up in the UV without some sort of top coat.
A good flexible stucco paint would probably do the trick.
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