If the flexing is just at beam ends into corner castings - re-rivet with monel or stainless rivets. If sloppy it would do no harm to include epoxy - this connection is not undone when you dismantle the boat, corner castings stay on beams! Do not exoxy castings onto the pylons built into the hulls or as you say you'll have a one piece boat! If this connection (castings onto pylons) is sloppy, first try tightening the trampoline lacing as much as possible, makes a surprising improvement. A more difficult problem is electrolytic action between the ss bolt through the pylons which will enlarge the holes allowing the castings to move up and down, you'll often hear a CLUNK just after tacking if you have this problem. If you have this problem I'll tell how I more or less fixed mine.