I'm 6'4" and my knees still sometimes hit the handlebars when I'm sitting on the crack. I'm often adjusting myself at stops to try and get as far back as I can. I even tried sitting on the passenger seat completely, which probably looked as silly as it felt.
I'm willing to spend a lot of money for a solution. But I already know there's probably never going to be a custom seat for this thing, so what else can I do? I don't know if stuffing the gap with something would really do much, I'm thinking of more of a soft seat cover that covers the entire seat. Plus, the seat isn't that comfortable, so I'd like more cushion anyway.