Hi domo,
Befiore you take on the challenge of designing and building your own mattress and ordering the latex layers and cover separately without the guidance or exchange options of a retailer or manufacturer that sells all the components together I would read post #15 here.
While there are too many unknowns and variables for me to provide any meaningful design advice based on “theory at a distance” without specific reference points of mattresses you have tested and more detailed phone conversations (which isn’t something I would have the time for)… mattress firmness/comfort levels in post #2 here includes some links to some of the theory and concepts that may be helpful.
If you are building your own then your best odds would be to approximate one of the successful designs used by a manufacturer or retailer that you have either tested locally for its suitabiity for your needs and preferences in terms of PPP or that was suggested by an online manufacturer or retailer for your body type and sleeping style and then come as close as you can to finding the exact same layers using the same type of latex in the same layer thicknesses and a very similar cover. You can read more about “matching” one mattress to another in post #9 here.
I would also be aware that Dunlop latex has a wider range of ILD variance across the surface and from layer to layer than Talalay and also that ILD’s are not directly comparable between different types of latex (see post #6 here). With Dunlop … comparisons with other Dunlop layers may be more accurate in terms of density than ILD (as long as the natural synthetic blend is the same).
I would also make sure that you are comfortable with the idea that if you make a choice of layers that are “less than ideal” and you don’t have a good return or exchange policy for the layers you purchased that the complete DIY project may turn out to be more costly than buying from a retailer of manufacturer that has component mattresses that has a good low cost layer exchange or refund policy.
It can be challenging and fun but also frustrating and sometimes costly and the “best” approach would be if what you learn along the way and the challenge and satisfaction of designing and building your own mattress is at least as important as saving money.
Phoenix