Hi Mexi,
If you are attracted to the idea of designing and building your own mattress out of separate components and a separate cover then the first place I would start is by reading option 3 in post #15 here and the posts it links to so that you have realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
I would also keep in mind that there are hundreds of different versions of memory foam throughout the industry and that memory foams that have the same density may still have a very different feel, response, and performance characteristics (see post #9 here and post #8 here).
The better sources I’m aware of for DIY components are listed in post #4 here but I don’t know of any that sell memory foam that is in the higher density range you are looking for so you may need to use lower density memory foam as a substitute.
Phoenix