Mattress for bed directly on hull

[bridgethorsepower]

The "aft cabin" in our 89 340 DB has a foam mattress right on the bottom on the boat. The bottom of the foam was shaped to take on that form and allow the top of the mattress to be flat (but it has worn unevenly over the years and is now following the shape of the hull). Any idea how to correctly shape the bottom on the mattress if doing a replacement DIY. I can't see trial and error being very efficient, shoving it into that room over and over again seems horrible. I'm comfortable getting the shape of the mattress correct, but not the bottom contour.

 

[Lazy Daze]

26 year old foam... yeah, it might be past it's expiration date. :smt001

What type of 'shape' are we talking about in terms of the boat? A wide, flat-bottomed "V" or something else? How much "rise or drop" are looking at? Can you post a picture?