Hi lax2luv,
You’re very welcome.
If you are attracted to the idea of designing and building your own DIY mattress out of separate components that are purchased from one or several different sources 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 want some guidelines on addressing the different quality of componentry, the durability guideline here offers up some good information.
If you’re looking for different options for componentry for a DIY mattress, you may wish to consider the members listed in the component post here, and mattresses.net is one of our site members on that list, which means that I think highly of them.
Mattress sizes are standardized in the industry (+/- 1" or so), so you shouldn’t have any issues with that. You may wish to speak with any mattress cover manufacturer you are considering and ask if they manufacture their covers to fit “snug” or “loose”.
You could use polyfoam, memory foam, latex, microcoils, or natural fibers on top of a pocketed spring unit. It would all depend upon your personal preferences in materials and comfort.
I look forward to learning about your progress, decision and any specifications if you decide to “tackle” a DIY project.
Phoenix