Memory foam is getting cheaper all the time. Home delivery through Amazon or many others is cheap or free. Buy one bigger than what you need and trace the outline of the desired shape with a sharpie. Even thick memory foam cuts like butter with an electric filet knife, even odd shapes and curves.
We replaced the mattress in our aft cabin with a 8” memory foam and added a 2” memory foam topper. It is the most comfortable mattress we have ever had. If they made the same mattress in Cal King size we would have it on our bed at home also.