I have found this $25 DIY amazingly comfortable. It's not that high and you can't really lean back but the added lower back/lumbar support really makes a difference.
I bought a cheap foam camping mattress and cut it in half lengthwise then rolled it up and duct taped it together and put it in a cheap stuff sack. It is rolled as if you were standing beside the bike not facing it. I then took a bread knife and did a little shaping on the end facing the seat for the best fit.
I used this on a couple of 800-1000 mile 3 day trips last summer and it makes a world of difference for me. I took off the chrome rails and used Rok Straps to attach it to the grab rails.