I have a couple ideas for you if they have not been mentioned yet. First, not to sound non patriotic or un-American, (please no bashing, it's just an idea, not for everyone.) Have you thought about trying to find some large fairly inexpensive American Flags & removing the blue square and stars? It would have to be a large flag but maybe? Second seems like a no brainer to me. How about simply buying a bolt of inexspesive white and a bolt of red material. Cut them into strips about 13 inches, which I believe is a standard size for striped circus tents, and then sewing them together. I am sure someone in your group has a sewing machine. If you are just using on the walls and not ovr head, you may not even need to sew them together. You could simply over lap them on the wall in the striped pattern.
Just two ideas that came to mind.
1986-1997 (Mutilation Mansion,) 1998 (Screamers Haunted House,) 1999 (Evil Intention Haunted House,) 2000-2001 Concept Creator/Business Partner (Urban Legends Haunted House,) 2002 Floor Plan Designer and Consultant for a (Haunted Barn) Owners had city challenges & were never able to open, 2002 Floor Plan Designer/Construction (Fright Nights Haunted House) 2003-2012 Now retired Owner (Deadly Intentions Haunted Attraction)