RE: not really seaplane question but...
Your Flutter concerns are going to come from the end of the tork-rod that is on the outside of the plane. The slop on the inside of the plane will give you sloppy control.
Let's deal with the flutter first. If you don't want to do micro surgery to dig the tork-rod out, you can help yourself a lot just by firming up the connection between the rod and the aileron. To do this, deflect the aileron one way or the other which ever will give you the clearest access to the hole where the tork-rod bends and enters the aileron. Now, with a very pointy CA bottle, just fill the hole with CA. If it is a reasonably tight fit you might be able to do that with THIN CA. If there is a lot of slop there you may need to use Thick CA. But start with THIN first. This will soak into the grain of the wood in the hole and not only harden it but it will make it water proof so that future lake matter will not get in there and soften it. One thing to consider is if there is a lot of slop there, be sure to center your tork-rod so that, when the glue dries your trims will not be too far out. While you are at it, go ahead and glue the tork-rod along its length that touches the actual aileron. Often this is also bare wood and you need to waterproof that too and if it becomes an integral part of the aileron, well, that is not a bad thing. Just be sure that the tork-rod and/or the aileron do not also become an integral part of the wing so be very careful with the glue. I would wedge a piece of wax paper in between the wing trailing edge and the tork-rod and aileron. Also be sure NOT to tip the wing and allow any glue to run down into the pivot trough. Since you are using CA as a filler here, just plan on it being an overnight cure and prop the wing up with the trailing edge down and the tip down and check on it in the morning. For me, this will mean a night for each side of the wing. You should end up with a very solid aileron-tork-rod connection. Regardless of the slop in the "trough", your hinges should support the aileron and keep it from fluttering.
Now, lets deal with the slop on the servo end. This will give you poor control because you are trying to move a lever that has a "mobile" fulcrum. I don't know if this plane has a tube for the tork-rod to travel through and pivot in. If it does, your soluton is simple. Just do as Jim said above, just pack epoxy around the tube to keep if from moving. Be careful not to get any epoxy into the tube. I like to use Stits Fiber Fill. It is more like a putty so you don't have to worry as much about it running into the tork tube. You can help by putting a drop of light oil or Vaseline on the rod where it enters the tube before applying any epoxy. Also see if you can slide the tube in toward the center of the airplane to put your support closest to the fulcrum. However if you do not have a brass or plastic tube for the tork-rod to ride in, you can do a little surgery and put one in. This will mean cutting away the covering. The part that is inside the fuselage should be enough. Just find a piece of Nyrod or a tiny straw or even one of those coffee stir sticks that looks like a tiny straw. A soda straw could work if you can slice it and reform it to the diameter of the wire. This will give you the hard plastic bearing surface that you need next to the tork-rod and keep the epoxy or CA off the rod. Whatever you find for a bearing surface, just pack it in the trough and support it with either epoxy or CA as you did on the other end. CA is always good because it wicks into the grain of the wood and makes it hard and waterproof. Whatever you do here in the inside of the plane, doesn't have to be that neat or pretty. Just be careful not to get glue on the rod. Work slow and thoughtful.
One other thing to consider is the force that is on the tork-rod end nearest the servo. You can minimize the stress on that pivot by lengthening the distance from the surface of the wing to the pushrod connection. The longer the distance from the surface of the wing to the pushrod connection, the less stress on that pivot point. I know it will also lessen your aileron throw but you can gain that back by lengthening the servo arm.
I hope this helps.