Before you go into big expense I think you had the right idea by trying an AirHawk air cushion. This will give you a good idea about what you might need before buying a custom seat. Normally they say to let almost all the air out of the cushion but for your situation start with more air and when you find a comfortable height have a friend help you measure the seat height.
Current Stable: 2014 XC, 2004 VStar 1100, PCX 150, 125 SYM Havana.
Born in the deepest jungles of India, I was raised by a friendly bear, until I was kidnapped by a pack of unruly baboons who taught me the finer points of swing jazz, all the while being stalked by an incompetent python and an evil but mis-understood tiger... Ummm, wait a minute, that's the Jungle book, no I was raised just outside of St. Louis, But...in the deepest jungles of a middle class neighborhood .