the only support you need is for the lower back. just tighten up the straps on your current seat until you feel it well supported. its posture and time on the water thats killing your back, very unlikely that its the seat. some lower back excercises and stretching will help you get used to the time. a lower back seat spring cushion from walmart that Ive seen some guys use, will also help get your posture right to support you upper body. it aint pretty. will rust in a years time, but its cheap and it works.
/bing