I took a large piece of ~2" thick Styrofoam (that was packing material for a flatscreen TV) and cut it to fit in the opening. I then glued (fabric glue) some dark brown felt fabric onto it...2 layers so as to provide cushioning and to enable the fabric to fill any little gaps when pressed up into place.
I took out 2 screws from the trim inside the shower holding down the skylight (one on each side center of the long side of the opening), and replaced them with ones slightly longer, with a piece of lightweight nylon strapping attached to each new screw (with adjustable clips so then can clip together).
So now, when we want the light (or while on the road...so as to not have additional 'stress' on the screws) I leave the Styrofoam light blocker just sitting in a corner with the straps clipped (to keep them from hanging down).
Now as we are preparing for bed I simply un-clip the straps, slip the Styrofoam into place, re-clip the straps, and voila...TOTAL blackout!
I actually made this for our Copper Canyon 5er, and fortunately it fit our Rockwood when we got it