I would scourer the larger fabric outlets for heavy clearance upholstery type material. Just make sure you check on a dark night that not a single beam of light is escaping.
How Far Away Can You See a Candle at Night?
http://preparednessadvice.com/survival/far-away-can-see-candle-night/#.U6GyJlOQ-NI
If you fly sky lanterns (aka fire balloons) you can see those for a very, very long ways away!
My wife owns a customer window treatment business.
Just go to a JoAnn’s Fabrics or similar and get blackout lining. Pretty cheap; more so than ANY upholstery fabric.
Consider using two layers, but NOT sown together; just hung one in front of the other.
EVERY pinprick lets light through, so no seams in the window area or light will leak in/out.
Back in the 70’s, we looked into cutting Homosote to fit the windows with the idea of covering the Homosote with quilted fabric stapled on. It had the bonus of keeping the room completely dark and having a huge insulation factor on the windows of that time. I am thinking of doing that now for the kitchen family room area. You just take them down during the day for light.