There are a few companies offering to custom build a mattress, I havn't gone down the road of pricing yet, but as you hinted could be pricey.
Our boat is our home so we thought why skrimp on our bed, current bed is foam with a topper and we have been very comfortable but the foam is old now and should be replaced, hence the looking around to see what is on offer.
As previous threads have comented Slats, foam and topper are pretty hard to beat.