Get a sheet of foam insulation board (AKA: "Insul-Board") and cut it just a bit smaller than the window frame opening. Use a putty knife to pack some strips of cloth or foam rubber into the gap between the Insul-Board and the jamb, all the way around.
Easy to do. Fairly cheap. Removeable and replaceable. Can be painted or coverd with black cloth to make it look nice. Done properly, it's 100% light proof.
You can get everything you need at Home Depot, etc. or, if you're like me, you can probably use some of the junk you already have lying around your garage.