Hi rsvoir,
You can read some of my thoughts about designing and building your own mattress in post #15 here. In many cases it’s a much more risky approach and can end up being more costly as well compared to buying locally or dealing with an experienced and knowledgeable online manufacturer. You can also see some of the difficulties involved in trying to “duplicate” or “approximate” another mattress in post #9 here and the posts it links to and some of the specs that can play an important role in how a mattress feels and performs in post #2 here. Even the cover can play an important role (often overlooked by people who aren’t familiar with mattress design and how every layer and component can affect every other layer and component in the mattress). In most cases it’s much “safer” and often less costly to deal with an “expert” rather than becoming one yourself unless the satisfaction that comes from the learning process itself of designing and/or building your own mattress is just as important as the cost of the mattress you are building.
If the materials in a topper are the same type and ILD of latex and they are the same layer thickness then they would be the same yes. there is no minimum thickness … only the combination of materials that works best for you. Post #2 here has links to some of the information on the site about mattress theory and design but they are only generic and meant to help the members here that want to know to understand some of the concepts and reasons behind different designs and don’t apply to any specific person.
You’re right that there is an overwhelming amount of possibilities in mattress design and each of them can have a very different feel and performance that is suitable for some people and not for others. Accurate and objective testing for PPP using the testing guidelines and your own personal experience is the “best” way to know which mattress is best for you. Failing that … more detailed conversations with retailers or manufacturers is the “next best way” where they can use “averages” to help you decide on which of the options and layer combinations they have available would work best for you. In some cases you could add to a mattress to “correct” it. In others it could involve replacing one of the layers with something else.
Latex foam is quite different from solid latex and is made in a different way and there are different “specs” that are important. There is more about the different types and blends of latex and some of the differences between them in this article and in post #6 here.
Phoenix