Came across this item for your seat belt that allows you to tighten the the lap section to hold you in the seat better but still allows the sash section to function normally. They also work with booster seats for those with little ones who travel offroad to help stop them moving around so much. Definitely less hassle than a harness. They are $95 here in OZ but i reckon you could get them out of the US for less than that.
http://www.cg-lock.com/index.html
How to stay in your seat better offroad
Moderator: Committee