a.twiddler
Veteran
It does seem that HP Velo have done themselves no favours regarding seats, not for original buyers, but for those who buy used, by giving too many choices. It might be worthwhile to look into cushions available for various bikes and trikes, to see what you could make yourself once you know what they look like, from some closed cell foam and velcro. It might not take much change to improve the comfort. I made a neck rest for my trike from a strip of plywood, some springy steel strip and a foam pad cut from a workshop kneeling mat, which I had lying around. That has a fibreglass seat base.
