I have that airplane OP. I havent built it, I am passing it on but I did go over the structure

1) Its very lightly built. So no speeding
2) Replace the stock pushrods. GP sometimes bundles the cheapest stuff. Go with 4-40 pushrods and ball links both sides. They are a bit heavier but will reduce flutter a lot
3) Seal the hinge gaps. Air leaking from below to above the hinge gap often contributes to flutter. You can use clear packing tape and only on the bottom side if you want

Remember its a 3D airplane, needs to be flown like one

Ameyam