My 2005 Yukon Denali has leather, but a number of years ago I replaced the seat pan and bottom foam. I made a mistake of putting my knee in the drivers seat and reaching into the back seat for something, do not know why I did not just use the rear door? I heard a "pop" and found there are wires that chris cross under the bottom seat foam and a few of the wires broke. So I ordered a new lower seat pan and lower seat foam. Not sure if these are still available?
I believe I pulled the drivers seat and found the seat covers were easy to remove, some fold over portion of the seat cover that has rigid plastic along the edge. Pretty easy, no tools required as I recall. Found that not only were a few of the lower seat pan wires broken, the side of the seat pan was also cracked from sliding over the edge of the seat while getting in and out of the vehicle. I am not a big guy, maybe 185. So swapped seat pan, then after looking at the entire set up, decided I needed something else under the seat foam. I had some sort of carpet scrap laying around, so I cut a piece of the carpet to line the bottom of the seat pan so the load could be distributed over the seat pan wires more evenly.
Put the new seat foam on and pulled the seat cover over the foam, flipped the sides back to lock the seat cover in place and put it all back together. This was probably 10+ years ago I did all of this. Set felt a bit better with additional support under the seat foam, kind of like putting plywood under a mattress for a bed I guess.
See what may still be available, otherwise you may need to find an upholstery shop that has some really high density foam that could build up a cushion and glue it together if layers are needed. It was a pretty straight forward and easy job as I recall.