How can you make a homemade spring tension rod that will handle 5 pounds of fabric over a span of 77 inches?

I am working at making a temporary window coverings for stained glass windows. I need to make decorative 'black out' type curtains and have chosen a heavy tapestry fabric to go with the period building. The windows need to be blacked out so that a projector screen is dark enough to be seen clearly and the flicker of light doesn't cause a visual strobbing distraction. When the weekly event is done, I need to quickly remove the curtains after the event and roll them up for storage the next week. Tension rods seemed to fit the need, but I can't seem to find them in the right length with enough strength to prevent sagging in the center, or outright falling down. I am not allowed to drill into the wood casement windows or in anyway mark them or the walls. Tension rods again seem the answer. I can only find Shower rods up to 72". Suggestions PLEASE!

Comments

[sarajo]

What about trying the command adhesive hooks? Since you can't drill, they will stick to the wall and then you slip the rod into the hooks!