Wrinkly flat panels

I've seen a few homebuilts with part of the fuselage panel that are flat
aluminum that are quite wrinkly and tend to "oil can" and make noise in
various flight attitudes. I haven't seen cross breaks used to stop this,
although it is used in duct work to stiffen flat panels. (cross breaks are
slight bends in the metal, done diagonally from corner to corner)

Another thought to reduce this noise is to spray urethane foam on the panels.
I know that this foam is combustable, but I figure for it to get on fire would
mean the pilot and passenger cabin is already engulfed, so it wouldn't really
matter.

What do you think?

thanks,
tom pettit