Tjingeling,
dusted off some old back issues of RCM today and found an old article from August 1976 where it is suggested that ailerons(or rudders) should be made thicker than the TE of the wing(or stab) to reduce flutter AND drag...! The article seems to be well researched and is supported by wind tunnel data. The author(W.D. Mitchell) summarizes at the end of the article:

"...make all your control surfaces about 12 to 15% thicker than the trailing edges of their respective wings or stabs. It´s the cheapest, easiest and most effective way to decrease drag and flutter, and increase control surface efficiency......"

Attach a figure from said article to illustrate and now I wonder why we have seen so little of this approach since it appears to be almost embarrasingly simple to apply. Cannot recall having seen any kit or design using this approach - why is that....?

The author also discusses the "usual" remedies such as sealing the gaps and mass balancing and he maintains that ".....mass balancing cures flutter if exact..."
............Cheers/Harald