Hi Buffet,
The first step in trying to approximate another mattress is to find out the specifics of the layers in the one you are trying to approximate. I don’t know the specifics of the Serenity but based on the description it would likely be close to the second version of the Pure Latex Bliss Pamper (see the specs here) except the top layer would be Talalay GL fast response (probably 21 ILD) and the bottom 1" layer may be polyfoam (like the first version). It’s also possible that it has the latex in the bottom layer (like the second version) or it may even be closer to the third version of the PLB mattresses here (which replaced the bottom 1" layer of latex and added it to the top layer). The bottom inch of foam would have less effect on the feel and performance of the mattress but a different thickness in the top layer would make a bigger difference.
In addition to this … there are many versions of memory foam so you would also need to choose a memory foam topper that was as close as possible to the characteristics and density of the memory foam topper you tried (you can see more in post #9 here about the ways that different types of memory foam can differ from each other).
The cover would also play a role in the feel and performance of each mattress so it would be important to make sure that you used a similar unquilted stretch knit cover…
There is more about duplicating another mattress and the different ways that may be possible in post #9 here.
If you are able to match the thickness, firmness, and types of foam in all the layers or the sleeping system and used a very comparable cover then you would have a close approximation in terms of design and “feel”. How possible this was would depend on whether the online manufacturer or retailer you were dealing with had layers and components available that were the same or very similar to the layers of the mattress you are trying to approximate and this would take a more detailed phone conversation with each manufacturer you are considering.
They may also have some ideas of different layer combinations that would “translate” into something roughly equivalent in terms of feel or comfort/support even though they aren’t the same design.
Phoenix