If you want to work in the basement, sealing the windows isn't really that hard.

Get a sheet of blue Styrofoam insulation board and cut it to fit into the window frame(s).
Use a putty knife to tuck some black (or dark color) cloth into the crack between the foam and the window.
Put some duct tape around the edges just for safety.

Windows sealed. Not a lot of work. Not too expensive.
And, it can be removed and replaced fairly easily.

That's how the window in my basement darkroom is sealed off.
You'd never even know the window is there if you didn't look for it.