I'm assuming that you mean in addition to a regular harness
to keep you on the boat,
You're asking about a harness
to keep you " in place" in a bumpy sea?
I read your earlier account of the trip. Let me ask..were you sitting or standing when steering? If standing..were you locking your knees in an attempt to stay put? If so, it's best to allow the boat to move under you like a surfboard. The boat is doing what it's supposed to do, keep itself upright, just go with it. If sitting it's a little harder to adjust, so I'll tuck in to a corner, or keep a hand on the stern rail. If it's really rough, we'll keep the shifts to an hour apiece....that's just me.. I'm sure others will have some sage advice...