Synergy has these lockers behind the seat on either side of the boat. They are great for can goods and spare parts. Here you can see the portside with the seaberth cushions and seat backs off and a bunch of stuff hung from the grab rails to dry.
Below is the starboard side with the seatbacks in place:
On Synergy there is a high sill on these lockers which keep things in place. They are quite deep since there is a seaberth above them. I have considered taking the starboard seaberth and converting it to locker with doors.
There is always and issue with how big the opening to these lockers should be. If the openings are small enough, then things will stay in the locker without doors in all but the worst knockdown. That is handy since you don't have a door to deal with, but offshore, it could be dangerous as heavy objects take leave of their bin. On some boats, the seatback has plywood inside the cushion that is held by hinges and acts as a door to keep things from flying about the cabin. That is something I have considered adding on Synergy if I take her offshore.