I'll try to help the best I can, first, here's some photo dimensions I snapped at the time. If I ever take it out again I'll make the center cut a tiny bit deeper for some additional wiggle room for the seat occupancy sensor. It works 100% as is, just a little more room for the pass thru wouldn't hurt I think.
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1 - The bottom blue piece is very high density floor foam play mat I originally got from walmart. I started here because it was rigid enough to ad some platform structure over the stock seat "springs" while still having some give. I think it's around 3/8" thick and doesn't compress easily
The top white ish layer is just a "high density foam" I bought off amazon. 1/2" thick. I figured better to have thinner supplies I could layer to build up than to try and cut it down. This foam is significantly softer and easily compressed than the blue foam.
I used 1 floor tile and only a portion of the foam I received.
2 - I accessed from rear passenger footwell. I did not unbolt anything and did not have to undo the front fabric clips to insert the additional bottom foam. That's only necessary if you wish to trim down the bolster foam. I'll try to show with this picture. You can try for access before buying any materials at all to see if it works for you:
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A - unclip the 2 x upholstry straps from the bottom spring
B - This will reveal the 2 x bigger clips underneath. Undo those, also from the seat springs, and flip both up and out of the way.
C - Orange = seat occupancy sensor. It's clipped onto the bottom of the seat. The wires don't need disconnected, the connection just needs unclipped from the seat frame for some working room. Pink = seatback cables, I assume heat and lumbar. Undoing these and flipping the cables out of the way makes slipping the foam in much easier or they will block about 1/4 of your rear access.
This will all reveal something like this:
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The metal bars are your seat springs, they clip up and over the exposed rear structural bar and clip in at the front. This is a bit finnacky but you can push the white pieces up and off of the bar, then slightly forward to drop the whole support spring from the rear. This makes a gap between the support spring and the bottom cushion and this is where I worked in the foam. There's a bit of back and forth between front and rear footwell to work the foam in as it can get caught on little bits here and there.
Then reverse. When slipping the white spring clips back into place be very careful of the front clip placement. They need to clip into the front of the seat pan (can see from front footwell) as the back clips are slipped back onto the bar. The front just needs to be lined up right before clipping onto the back bar for it to fall into place correctly. First time is a pain, after that you'll get it.
Re-install spring, re attach wire harnesses, re, clip wide fabric clips, re clip short fabric clip, done.
3 - Rear. The vast majority of it is done from the rear. From the front I unclipped the wire 2 x harnesses, unclipped the occupancy sensor from the seat frame and helped guide the foam in / pulled the foam in with the spring dropped when it was getting caught up pushing from the rear. Don't push blindly too hard. Also from the front make sure that seat occupancy sensor is clear and free / not pinched or routed in a way it will get damaged. Mine is looped forward a bit and dangling straight down when done.
4 - I'm mainly curious if additional foam would offer any further plushness. But I have 2 concerns: 1 - I'm pretty happy with where it's at right now and 2 - I don't know how much vertical foam you can install before you start running into complications re-installing everything. Right now I didn't have to mess around with anything, all fabric clips re-install as they should even though they're a bit more difficult to stretch out during re install.
A massive wall of text, sorry. But I hope it helps.
I actually have enough material left to make about 3 more of these as pictured. I'd offer to make one up and send it but cross border shipping with it's size is likely cost-prohibitive. I wouldn't want to fold up that rigid foam. Also tough to guarantee success, it works for me at the moment but I can't say for certain it will help with the discomfort you're feeling.
I have gone from nearly regretting the car and being afraid to tell my wife I might need to ditch it to once again enjoying it.