That's maybe not entirely right. The seat heaters appear to be attached to the seat skin, not the foam.
You can get your hand in the space on top of the factory foam cushion and under the skin/heater, and you can fit foam in between, on top of the factory foam-but...
In order for the skin to be pulled tight against the cushion and always stay in position, many of the seams visible on top of the seat have a small nylon rib sewed onto the underside of the seam. Then there are metal clips that clamp onto these ribs and attach to anchors, like buttons, cast into the foam.
What's this means is that you have to work in small areas between the metal clips, and it's much more fidgety getting any foam you insert sized and placed just right. When you go on top of the foam cushion you can affect directly the softness, the plushness of the seat, but that's A LOT more work.
By inserting under the existing cushion you affect the way the seat supports you, which met my needs. It's great if want to play with foams on top of the cushion too, but it will address different parts of how the seat feels.
This leads to the topic of making a foam kit... This is convenient, but... I'll bet if we all got different materials and played till we were all happy you would end up with multiple approaches and different "feels". It's easy to make adjustments, play with different options.
To start though, keep in mind there are some realities that should be remembered. Whatever you put as a bottom layer, directly on top of the spring, is going to take a lot of punishment. A soft open cell foam, or memory foam, may get ripped to pieces pretty quickly by the springs. This is why I suggest something durable for the base layer.
I used closed cell tough foam here that barely compresses as a base to reshape how deeply my tailbone and hips can compress down, and therefore not press me into the side frame that's painful to me.
Then it was a bit too firm though, and I could feel the Ridge between where I had 1 layer of closed cell foam and where I added more, or had none.
By adding memory foam over the closed cell layers and under the factory foam cushion, I filled in the ridges as it were as the memory foam gets highly compressed between layers, but fills the gaps where they occur.
If I used memory foam only I wouldn't have gotten the support I wanted, and the springs would rip up the memory from in no time.
But that's what I like, you may prefer more memory foam, for even support, more closed cell for more reshaping of how the cushion rests against the frame, open cell medium density foam for more bounce, or foam on top of the cushion make it more plush, but maybe lumpy.
After I got my driver's seat where I wanted it, I duplicated it on the passenger side, and that worked great. A kit then could give you fairly repeatable results, but you'd want to have a choice of sitting on several kits to pick what works for you. If we can't learn from the fact that different people with different body types have the full range of satisfaction to dissatisfaction with the oem seats, it Stands to reason that there is also no one Size fits all approach to the fix either.
As you play though remember that whatever you add will have to be durable, it may affect the sensor in the passenger seat, it will have its own properties for collecting odors, flammability, what gasses it gives off as it burns, and so on. You mod your car and you own your results. If knowing the flammability rating of your foam sounds important to you, then own it. If not, that's fine too but make conscious decisions. If what we install makes something like the leather or clips wear faster, it was your mod.
I'm giving some of my unused materials to a friend for his Bolt, but I've known him for years. You couldn't pay me enough to sell kits though. I don't need some ambulance chaser and a crazy widow chasing me down if the passenger airbag sensor is affected or if a seat heater makes a random foam burst into flames. Lawsuits have nothing to do with realty, and I'm too old to lose everything...
To me this is why GM should step up. Every dealer should have perhaps 3 demo seats, each with a different kit installed. Each of those kits should be fully vetted by the engineers and material scientists who do this stuff full time. Until they do, I'll take personal responsibility for my mods, and answer any questions about how and why I made my choices, but I don't endorse any of this for any of you.
Sorry for being paranoid, but there are complexities here, and GM needs to step up.
