I was trying to solve the same issue you have. I wasn't too keen on building a frame for the illusion of a boarded-up window. Then I was at Spirit Halloween store and saw this:
So, that got me thinking. I could make the wood using the faux wood made out of foamboard (
Spookywood) found at Spookyblue's site:
Fake Wood From Foam Boards - SpookyBlue.com
Then attach it together with a string on each side. Finish by attaching some fabric backing (
burlap, cheesecloth or jute netting). Hang by nailing in two nails at the top of your window frame and hang up. I think that this would be good enough to give a decent impression of a boarded up window with a minimal amount of work.
Ran out of time this year but I'll do it next year.