Nassaw, the yachtie way is to make custom hatch
covers of Sunbrella to keep the UV from degrading the plastic. You can only protect them by covering them from the outside. That also protects them from abrasion by blowing sand and salt
crystals, both problems by the sea.
The USAF protects the glazing on mothballed aircraft in similar ways. Covered from the outside with a special latex-like coating, sprayed on.
Anything on the inside won't block the UV and, as you suspect, will only help cook the plastic.
Don't use ay type of adhesive
(i.e. self-stick vinyl) that has a petroleum-based solvent in the adhesive
, as almost all of the do. Petrochemicals degrade the plastic as well.
For a cheap
& dirty approach you could probably paint
them over with a water-based latex frisket (from any art supply house, sold as masking material for stencils, etc.) and then press a dark canvas
, etc. into the frisket, bonding it on. Then just peel it off when you get back.