The yellow stuff is an extra piece of high density foam. I've also put in some cable to hold everything in place.
Ford F-250 Bench Seat Repair
This is the completed repair. The crushed original (orange) padding has been supplemented by yellow foam from some kneeling pads (for gardening?). Zip ties and some cable keep everything in place.
Yellow foam restores the height of the padding. I can't really remember, but it looks like I also put a green towel in there to firm things up.