Hi susu123.
Attempting to recreate the feel of a sectional sofa can prove very challenging even for the most experienced mattress manufacturers and furniture retailers. A sofa-mattress feel matching is even more difficult than a mattress-mattress matching (which more often than not proves to be a futile exercise). You might find it helpful to peruse post #9 here which describes several ways that one mattress can “match” or “approximate” another one.
Crate and Barrel has 35+ collections and many models each with different configurations, materials, and components. Every individual layer, component, construction/design (depth, length of the chaise, thickness, firmness, cushion number, etc) in your sofa (including the cover, FR barrier, any quilting material, and of course all foam layers and support) affects the feel and response of every other layer and component and how it “responds as a whole” to each individual sleeper. This is to say that you’d need to study your sofa, carefully assess the design, and find the specs of each component in order to determine if you can create a mattress that approximates its feel and be a good “match” for you in terms of “comfort”, firmness, and PPP(Posture and alignment, Pressure relief, and your Personal preferences)
I’d keep in mind that any slight change or deviation from the “model sofa” will be amplified by any other unmatched component or specification in your DIY and will modify your experience and the feel of the target mattress as a whole. There are also several factors that can affect how soft or firm a mattress (or an individual layer) feels besides just the exact specs, density, and ILD of the foam used (see post #4 here for more information )
The best you can hope for in such an endeavor is to find something “in the range”, but only your own careful personal testing along with several detailed conversations with an experienced and knowledgeable manufacturer/retailer may have any chance to get you a little closer to a mattress that replicates your sleeping experience of your current sofa.
Phoenix