I bought one of those inserts that is reflective on one side and like sheeps skin (not real) on the other,e but it never stays in place due to the crank handle. So I used a left over piece of mattress topper and some white masking tape and taped it to the ceiling.
Here in Texas there isn't a whole lot of days where I want that vent open, so I just leave it like that so block out the light.
If temporary, could use painters tape to be sure the tape comes off easily.
2018 Primetime Crusader 319RKT
2013 RAM 2500 4X4 Laramie CrewCab, 6'4'' bed, 3.42 RAR, 6.7L Cummins Turbo Diesel, Payload 2547, Max tow 17,480, PullRite Superglide