Hi Elvis_Presley,
Welcome to the Mattress Forum!, King! :lol:
There’s some good sample information from the Polyurethane Foam Association on common IFDs of polyfoam used in seat cushions here.
What you’re describing here is more of a topper application, and this post (and the multiple posts it links to) provides a wealth of information about toppers, how to choose them, the different topper materials that are common, and some sources for toppers of which I am aware.
This would be more along the lines of creating your own polyfoam mattress. The thing to realize is that the cushions on your couch will feel different than making your own mattress. Besides the difficulty of matching the foam density, thickness and ILD, you also have the surface beneath the foam cushions (quite often some sort of a flexible surface) that contributes to the overall comfort, the cushion covering material, and you have the backing of the couch, which also alters the sleeping posture. If it’s just an issue of your current mattress being too firm, your better option will generally be adding a topper to a current mattress that is still in “good shape”, as you’ll most likely have a better chance of obtaining good support/alignment, durability and comfort with such an arrangement.
The dacron wrap on top or around a seat cushion can add a small amount of extra comfort, but I generally wouldn’t order a piece of polyfoam with this glued or stapled to it, and instead would look at topper options, and there are fiberfill ones available should you want to go that route.
If you’re considering purchasing raw foam materials, some better sources of which I am aware for components are listed here.
Phoenix