Finally decided to just sheet the darn things. I sheeted the bottom first and noticed a significant improvement in the stiffness. Was going to leave the top open and just cover it, but a flying buddy came over today and said the top sheeting would probably make them stiff as a 2x4. He was right.

Here's a couple pics. You can see one is fully sheeted and the other is still open on top. The fully sheeted aileron is very stiff. Almost no twist with my hands. The one that's only sheeted on the bottom is still quite flexible. I'll be curing that problem here in about 20 minutes.