Obviously the best au jus is going to contain drippings from the meat captured as it cooks. Easy to do with roasting pans in the oven, not so easy on a pit.
Some time ago I tried one of those packets of dry au jus mix - basically a boullion cube or two that has been pre-pulverised - overly salty and dubious beef flavor. If you've got the time, I'd suggest roasting some beef bones with onion, then simmering to make a stock. Perhaps some other folks will have ideas on using packaged beef broth and such.
Oddly enough, I'm planning on a batch of beef chili this week, and tonight I baked and boiled some beef back ribs to start the process. Here they are getting ready to flip before roasting another half hour or so:
Knowing the end product is chili, I've laid them on a bed of onions and pasilla peppers. The roasting is done, they've been simmering for a few hours now, the kitchen smells delightful.