Morning. The only way to get new cushions for your boat is to be there with an upholsterer. They will measure everything in situ because boat cushions/mattresses are such odd shapes. Alternatively, you could do what we did. We bought a good quality plain foam mattress. Once it had expanded to the full thickness, we laid the old bed cushions on top (on the landing stage) and then drew round them and cut the foam ourselves to the correct shapes needed.