When I was looking to redo the V-Berth on my boat, I researched mattress options, and ran across several 'under-mattress' systems and ended up going with the Froli Modular SleepSystems.
The big challenge is ventilation under the mattress.
- Affordable and well made
- Easy to follow instructions
- Easy to assemble and use
- Comfortable and adjustable
- Great ventilation.
As others have noted, it can be a wrestling match with the Froli to get to the lockers under the cushions in the V-Berth. What has worked for me so far is keeping the overall pieces small enough to handle, instead of creating one large connected piece, I have three smaller pieces: One on port, one for the bench starboard locker cushion, and one for the center key piece.
It's not like sleeping on a king sized Casper mattress, but it's comfortable. I was able to keep the V-Berth cushions to 3″ and still have a relaxing berth and a good night sleep.