Hi dip,
Rocky Mountain Mattress is a manufacturing member of this site which means I believe they are among the best of the best in the country in terms of quality, value, and service. This page goes into more detail about the manufacturers and retailers that are invited to become members and are recommended by the site.
As you may know from reading the mattresses section of the site … memory foam density is the most important part of the quality/durability of a foam. Having said that … there are hundreds of different memory foam formulations even if the density is the same. Some of the differences between different memory foams are in post #9 here and there’s more in post #8 here as well.
There are really 3 different ways to “match” one mattress too another (more about this in post #2 here). Since your custom mattress would be using similar layering and quality/density of materials but materials with slightly different properties … you would be using the third method where the manufacturer themselves would be the ones that know their foams the best and specifically how a certain design may compare to another design that is familiar to them.
So in this case … I would trust Rocky Mountain to be able to do what they tell you they can do and I would let them design the layering and tell you what it would be. This would be much more accurate than any ideas I may have about what it “should be” because I don’t have the same knowledge and experience with the specific materials, components, and designs that are used in their mattresses that they do. It would likely end up similar to the rhapsody in terms of layering but may also have some differences that take into account the differences between the components and materials they use and those in the Rhapsody.
Hope this helps.
Phoenix