Hi H8tobogie,
There’s not too much that I can add to the excellent suggestions that dn has already mentioned.
In most cases I tend to discourage trying to design and build your own mattress from separate components from separate suppliers because there is no formula that you can use to predict how the specific design you are considering will interact with your own body type and sleeping positions. Each person is very different in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) and what works well for one any not work nearly as well for another even if they appear to be very similar in terms of their body type, sleeping style, and circumstances.
Some of my thoughts about building a complete DIY design are in post #15 here and in post #2 here. There are also some links to some of the theory involved in mattress design in mattress firmness/comfort levels in post #2 here but these are only generic and useful as concepts and are not specific to any individual person.
Many local manufacturers can make custom builds and don’t include everything they have available on their websites. If you do decide to go in the direction of designing and building your own mattress on your own then realistic expectations of success and the chances of having to replace layers and the costs involved (most individual layers have some cost involved or are more costly in the first place if there is a return policy built into the price) are important and one of the most important steps would be as much personal testing on as many different combinations of materials and designs as possible so that you have personal reference points for what specific combinations tend to work best for you. This can involve a fairly lengthy and steep learning curve if you are building a mattress where you haven’t personally tested a specific design or are using material combinations you aren’t personally familiar with.
Phoenix