Check out black plastic tarp which is used for covering crawl spaces. It's so cheap it's almost disposable. If you are printing at night, it's obviously much easy to block out the night than the day.

Get a putty knife and use it to tuck the fabric in wherever light is leaking through. You'll be surprised how easy this is once you've got the bulk of the light blocked out. You can kind of push it into the cracks by the sash.

Alternately, get some wide masking tape and just tape it to the wall.