How to waterproof outdoor storage bench?
For the lid, you'll need to support the decking as frequently as it is supported on the deck itself.  If your deck joists run every 12" or 16", you'll need to replicate that or the lid will sag. The photo shows benches that are sixty inches long.  You diagram show one support in the middle, leaving a 30" span.  At summer temperatures, it will be sagging within a week even with zero use.