I used to have an insert that went into a (large) bathroom window, and it worked very well. The key is to make it with a single piece of plywood that overlaps the window frame/opening, and an insert frame glued to the cover board that fits into the opening.
I made the insert frame just a smidge loose, and made up flaps from black masking tape that filled the space. I covered the joints between the insert frame and the main plywood piece with the same (two layers, in both cases), and before taping anything, i painted the whole thing, inside and out, with Krylon Ultra Flat Black. If you switch from tape to weatherstrip for the gasket between the insert frame and actual opening, you can fill a fair amount of roughness, but key here is that most of the light sealing is from the light having to make a sharp corner in a narrow space, with very flat black surface on at least one side. With this insert, weatherstripping on the door frame, and a towel in the gap below the door, my darkroom would get dark enough and my eyes would adjust enough that I could see my hands in the light from the luminous hands on a Graylab darkroom timer, when it was turned against the (light wallpaper) wall. I tray developed ISO 100 film on several occasions, and loaded film being pushed to beyond EI 5000, with a window insert I could put up and take down in two minutes or less.