Here's a photo of my window plug I finally installed yesterday. Primary goal of this was to help block sound rather than light (I live near black people) so it was designed to be extremely heavy, thus the handles, which helps block low-frequency sounds such as rap music and cars without mufflers. The inside is 3/4'' MDF which fits almost perfectly into the hole for the window. The outside is 1/8'' hardboard which is flush at the bottom, but extends out onto the window frame an inch or so on the other three sides. Adhesive-backed closed cell foam was applied to the bottom of the MDF and to the top three sides of the hardboard to help seal it, and then it was screwed to the wooden window frame with several small screws. Blocks all light, seems to help with noise, although the room has another window that allows for noise pollution from another direction. It is already blacked out about 90% and I do want to allow at least a little light into the room.

MDF and hardboard both outgas noxious chemicals. It requires completely painting both pieces before joining them to fully seal them and keep the room odor-free. Probably not a bad idea to let them outgas after cutting but before painting-- as well as after painting.

Big part of the problem for me is that my house was built in the 1940s and the walls are almost certainly uninsulated plaster, so there is probably much more sound-absorption through the walls themselves. But glass is a notoriously poor insulator of both sound and temperature, so this is a start.