Just a piece of foam, but perfectly suited and sized
This thing is so perfect.  Yes, it's just a wedge of foam, but it's shaped just right and is a perfect density.  I have it on the drivers seat of my truck so I can see the front of the truck a little better, and I positioned it under the seat cover.  It disappears from sight as it is slanted so as to just eliminate that dip at the back of the seat.  It's comfy but doesn't flatten when you sit on it.  I love this thing!  I'm sending one to my daughter.  I feel really safe being up a bit higher and if someone else gets in my truck they don't act like I made them sit on a big pillow.  They don't even know it's there.  Also, it doesn't raise your knees/thighs up (just the butt) so that works great.
Rating: 5 stars