If you are looking for a more affordable route, buying a foam “bed in a box” and cutting it down to size with an electric bread knife is a solid option.
Ava and I made our mattress with sheets of foam we bought at a bedding store in Costa Rica. It took one full day to cut the foam sheets into the correct V berth size, contact cement the two densities of foam together, and sew a cover over it with the scrap fabric we had.
Starting with a bed in a box, all you would have to do is cut to size and have someone stitch or modify the cover.
I think the beds can be had for under $400 on Amazon.
We made our bed out of 3” of hard density and 2” of medium density foam. It’s so comfortable that sleeping on land beds again has been a real struggle.
Side note: if you decide to put seams in the mattress to access the under berth storage, try not to put them in the center of the bed if you can help it. We decided to cut the mattress into three sections with seams athwart ship so that a couple won’t notice the seam when sleeping in the berth.
-P