Hi litesleeper,
The types of exchangeable layer mattresses you are talking about are becoming more and more popular. They have been available from smaller manufacturers for many years but are appearing in mainstream choices as well now although these are being sold at highly inflated prices. These types of mattresses will often have a choice of layering that can be customized to the needs and preferences of different people and will also often offer 'side to side" split layering that can be individually customized to the needs and preferences of two people on a mattress. There is a list of manufacturers that are members of the site in post #21 here that specialize in online sales and many of these offer what are called either DIY mattresses or “choose your own layer” mattresses with various components and layers inside a high quality zip cover. Those that do this will also usually offer “layer exchanges” so if someone makes a choice that isn’t quite right then just a single layer can be exchanged for a firmer or softer version to fine tune the mattress closer to that person’s ideal. Of course an additional advantage of this type of mattress is that if the upper layer wears out first (the top layers are the ones that wear out the fastest) then the mattress can be unzipped and just a single layer replaced.
In a similar way … many people will buy a base mattress with a single firmer core or at least very little foam in the comfort layer as a base and then add their own topper as a comfort layer that is suitable for their body type and sleeping positions. This way the basic mattress becomes the support core (generally firm) and the topper is the comfort layer. This also has the advantage of being able to exchange the topper which will usually wear out faster than the support core without having to exchange the whole mattress.
There is a lot of information in the mattresses section of the site which goes into more detail about materials, different body styles and different sleeping positions, and about various types of construction or putting the layers together. These are just guidelines though based on averages and a better “starting point” than an “ending point”. Some height/weight guidelines are here and some sleeping position guidelines are here (just to save you searching through the many pages in the section). There are very few people who are really average"" though and a far more effective approach would be to learn just enough that you can work with a manufacturer and ask better questions and better understand the answers.
The choice of materials, components, and layering of a mattress can be amazingly complex because every layer material, firmness, and thickness will act differently and in combination with every other layer. It is a combination of technical knowledge and intuitive knowledge and a combination of art and science. Even manufacturers that have been in the industry for decades and are part of a family of multigenerational manufacturers are still on a learning curve. Different body types and sleeping styles will also interact with the same mattress in many different ways even if they are similar. Different people also have many different levels of sensitivity to various designs and different preferences as well. The variables can be very frustrating, challenging, or rewarding depending on the type of person who is designing and building the mattress and the amount of time and effort they are willing to put into the project.
Post #15 here outlines some of my thoughts about buying locally vs with the help and guidance of a DIY online manufacturer vs trying to do it all yourself by putting together different materials without the guidance of a manufacturer who knows how all the materials may interact. Post #36 here may also be useful.
If you do decide to go in the direction of building your own mattress through various sources as opposed to using a DIY construction through a knowledgeable manufacturer … then it would take a lot of research into mattress materials and local testing to make sure that you end up with the layering, materials, and components that may work best for you. In some cases it can become far more costly and difficult than working with a quality manufacturer who already offers the same thing all in one place and can give you the benefit of their knowledge and experience with the specific materials that they offer.
Phoenix