available in many configurations here: Dierks Anchor Systems
A friend has a similar configuration bolted to the rear of his cat. but your seat arrangement may hinder this type of mount.
Maybe the above link will give you ideas but assuming the rear portion of your seat bar sits a ways behind your floor (as it appears) Osprey's idea of pivoting sailboat pulleys may be the simplest. Hang one directly down from the rear of your seat bracket (middle), another on the corner (where front rail of seat bracket meats side rail) and one on the cross bar above your foot bar (also near the side rail) and you'd have a pretty fair system.
edit - link to pulleys - many more where this one came from: http://www.p2marine.com/catalog/prod...dYQaAroM8P8HAQ