You could just redo your seat with something like the kit from Seat Concepts or I believe there is another seat company out there with products for Alta's
Easiest thing would be to take the seat to a upholsterer, and have him redoo the inner material to your liking.
Add material or take some out, replace some with a softer foam, and so on..
We have such a guy in Israel, and all short/tall riders use his skills all the time.