Hi SamWhoNeverSleeps,
Deborah at Diynaturalbedding is very knowledgeable and helpful and is also a member of the forum that has taken the tiime to provide some great advice to the members here as well (you can read her posts here). I certainly wouldn’t hesitate to deal with her.
If you are attracted to the idea of designing and building your own DIY 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 (and option #1 and #2 as well) so that you have more 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).
If you decide to take on the challenge then I would either use the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of PPP as a reference point (the same type and blend of latex in the same thickness and firmness levels and a very similar cover which can also make a significant difference to the feel and performance of a mattress) or use a “bottom up” approach (see post #2 here).
The better online component suppliers I’m aware of are listed in post #4 here.
Phoenix