I would do what you can on the top to help stiffen it up. But for me I would build a deck in the bilge for the bottom of the bait well to rest upon so most of the weight would be supported by the deck in the bilge instead of the well hanging in the air. Thats how I did my deck and I get no flex. My livewell now is a support for my deck instead of my deck being the only support for my livewell.
FellowShip
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Just for the Grins
Official Antique Classic SeaCraft Owner