Hi AT,
I haven’t seen anyone on the forum build a DIY innerspring/latex hybrid (most people who build DIY mattresses use different combinations of foam layers) although there have been a few that did mattress surgery on an old ininerspring mattress that had developed soft spots or was sagging and then replaced the foam that had softened or broken down with latex.
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 springs, the foam layers, and the cover) as closely as possible or use a “bottom up” approach (see post #2 here).
The biggest challenge with building a DIY innerspring/latex hybrid mattress will be trying to “match” the springs in your reference mattress and the cost of shipping for the springs as well (which can cost more than the innerspring itself).
If instead of building a DIY innerspring mattress you are thinking about just buying a firmer innerspring mattress with thinner foam layers and then adding a softer topper this would be quite different from building an innerspring/latex hybrid.
One of the challenges of buying a firm mattress and then adding a topper when you can’t test the combination in person is that choosing a suitable topper that is a good match for both your mattress and for you (different mattresses will affect the choice of topper that works best) can be almost as challenging as choosing a mattress that doesn’t need a topper in the first place because the only way to know whether the combination will be a good “match” for you will be based on your own personal experience (see post #2 here). It can also be more costly than just buying a mattress which includes the same latex layer inside the cover without the additional materials that would be in your base mattress that you may not need or that may be softer than what would be ideal for a transition layer. At the very least I would make sure that the topper has a good exchange return policy so there would be less risk of buying a topper that doesn’t turn out as well as you hoped for.
If you choose a suitable topper and the mattress/topper combination turns out to be a suitable “match” for you in terms of PPP then it does have the advantage of being able to replace just the topper without replacing the entire mattress if it softens or breaks down before the upper foam layers in the mattress (the upper layers of a sleeping system will tend to soften and break down before the layers underneath them) or if your needs or preferences change over time and a topper can also help extend the useful life of a mattress underneath it.
Phoenix