I use and recommend a double sling system. They are also called Biathlon Slings because the Target Rifles used by these athletes are very heavy and require a double sling system.
I like the Vero Vellini Double Sling for its quality, padding and comfort. They sell for about $80. dollars. It's a qick release sling, so it can be removed quickly when necessary.
Len Backus sells a double sling in the gear shop that is much less expensive, if the price point is a factor.
Here is a link to the Vero Vellini Sling that I use: Pioneer Research - SeaLife underwater cameras, Vero Vellini slings and straps, Vion hand bearing compass
Safety dictates never carrying your crossbow loaded while not in your hands. Can you imagine what might happen if the limbs fired while this was on your back and they hit your head or some part of your body? 155 foot pounds lbs. of energy is a lot of energy being let go all at once.