One choice is Allen Silver's retrofit Crotch (Anti-Submarining) straps
------ $30.00. It is mentioned on this site. http://silverparachutes.com/acrobelt/.
As I remember it, it is unside down Y shaped, ties into the existing
lap belt bolts, and then into the center seatbelt release. Give Allen
a call to see if it will work on your ship - great guy.