Sorry, crab
it's absolutely
unsafe ! And it's still impossible with an unconcious patient. We have seen the result!
How much time would it take to get every casualty into a 3 point harness (especially if they are unconcious) when a double strop lift can be done in 20 seconds?
I recommend to inform about rescue equipment. A lot of different really safe products are available. It needs not more than 10s to strap a also unconcious patient in a triangle rescue nappy. You have to close only one plug.
a Nato strop is still OK but you should use a second one under the knees so the casualty comes up horizontal
Could it be that we have a new century? Such improvisations be good for nothing for a
professional rescue team. And if you maintain that a unconcious patient could be lifted up safe with your "method", i'm irritated, thinking about if you are an experienced rescuer?

What if your patient have broken legs or arms or shoulders? What if he is wet, slipping trough the strope?