You are sitting upright on a rather narrow seat that is mainly a thinly padded sheet of metal. They still make suspension/springer sissy bars or can use a suspension seatpost. Going to have to do a custom reuphostery and add padding like others have mentioned. Finding where you sit on the banana seat and you could pound that section with body tools to form it to your sitbones and then take it to an upholstery shop to add the necessory padding for more comfort...maybe a diamond tuck for some style. That being said, it might be better to go full custom and have a wider and thicker padded banana seat made ala motorcycle chopper style. Seen that done back in the day...mostly with the 24" wheel varients.
Before my bike got stolen, a stingray/manta ray was a short distance bike, not a bike I wanted to ride the 10 miles to the beach. The pain was too much and I only had a single speed which was fine to visit school friends and hit the local stores.