Have the seat rebuilt to you liking, or do it yourself. You can add/subtract foam from a seat to make it more to your liking without any experience. If you need to remove support an electric knife cuts foam like butter. If you do not want to cut the original seat then remove the stock piece, (that is hurting your back) duplicate it, leave the cover off of your seat, cut and adjust until it makes your back happy, then when you are satisfied reinstall the cover. If it is something you don't want to tackle yourself look around and you are bound to find a shop that does seat upholstery, if they don't do foam they should know someone who does.
Clif

Traded for above-Custom intake, 2 1/4" ehaust w/Hedman Pack (pn 25005)