Hi mattress maven,
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 (Posture and alignment, Pressure relief, and Personal preferences) as a reference point and try and “match” every layer and component in your reference mattress (including the type and blend of latex, the thickness of each layer, and the firmness of each layer as well as the type of cover as well) as closely as possible or use a “bottom up” approach (see post #2 here).
Other than this … there really isn’t much I can add to the comments from the two members that have already provided some great advice.
If you are uncertain about the thickness of your cover you can always order it last when you are fairly certain about the thickness of the layers that you will end up using in your design.
Phoenix