I see I am gonna have to actually engineer one for you..
However, depending on the type and model, I dont see where it would be all that hard. Even if a different type from what I showed was used and attached behind the seat to a hard point. Whether you used a seat with a shoulder belt hole, or used the side wall as a shoulder sash guide anchor attachment
point (look at how flimsy this is on your car), it could be done.. all it takes is a little research, time, and few modifications.