Not sure if you are asking about a 14 x 14" square crank up vent skylight or the skylight in the shower?
My wife made pop in cushions for both. We install them when we are not using the shower or the roof vent.
These help keep the sun heat out and heat in in cold weather. They also help cut down the sun UV rays that turn ABS plastic showers yellow over the years. See pics below. I cut the foam she bought at Joann's Fabric store, and she did the material sewing part.
Here is he crank up vent. This is not the bath one, but we have 3 of these vents and the material is different on each to match the room better.
Here is the shower dome one. This was a little more involved as the dome is curved and we cut the foam on an angle to fit up into the curved part. The\n used 1/8" shock cord to hold it in place. The shock cord, unhook on one end and then we move it still attached as the other end and fling it outside the shower.
Cushion hooked in place.
Cushion cut on an angle
The shock cord part and clips less the cushion
The clip is a nylon 1 hole electrical strap with part of the strap cut off so the shock cord and slip onto the clip.
Hope this helps.
John
2005 Ford F350 Super Duty, 4x4; 6.8L V10 with 4.10 RA, 21,000 GCWR, 11,000 GVWR, upgraded 2 1/2" Towbeast Receiver. Hitched with a 1,700# Reese HP WD, HP Dual Cam to a 2004 Sunline Solaris T310R travel trailer.