Remove play from hull to beam joints.
Cover the beam joint area with parcel tape/polythene/food wrap and bed each beam into a layer of resin in the beam box. Place a washer over each bolt between beam and hull to allow a uniform layer of resin to remain in the joint and tighten the bolts lightly. Wipe excess resin whilst wet and let cure. Once cured undo joint and remove beam tape film etc. and clean up joint. Also, if you remove the washers at this point you are left with a convenient recess for sealant when you reassemble the joint. Repeat on all four corners. Once done properly remaining flex should come from lack of stiffness in actual beams/hulls. This should be done regardless of what else you try and should make a noticeable difference.

Cheshirecatman