It will be open, the cooling air will go through there, i may put some aluminum tape or similar on it though, it cant hurt.

My last CAP i built 2" wide ailerons, they are built up similar to the tail surfaces. You have to move the trailing edge in order to keep overall chord on the wing the same which is a bit of work (not sure if its worth it if you have built the wing already). You could use aileron stock i guess but the built up is not hard to make.

And almost done my building table, so i can soon start the wing.