So, on the one hand, you've got good heat control with a propane grill. I use mine to smoke sometimes. On the other hand, you're going to have a hard time getting that super smoky flavor on the brisket.
Here's what I did to mine to make it more amenable to smoking:
1) I dumped a couple of bags of lava rocks in the grill to smooth out the flames
2) I used a hole saw to drill a hole on the right side of the grill lid to encourage smoke to flow out that way
When I smoke with it, I use one burner, on the left, and keep a thermometer in there to assess temperature. Above it, I put the wood.
When things go well, Smoke rises out of the hole made with the hole saw, temperature is even, and I'm happy. Unlike a weber smoker, you're going to want to rotate the meat using this method. Maybe it would have been better to put the hole in the middle, and use the two outside burners, but this would be too hot for my usage in my grill.
I don't personally use a foil box, I just put wood right on the left side of the grill, or on top of the lava rocks, and occasionally blow out the ash.
p.s. If you take your meat out a while beforehand, get it up to room temp, and then put in the path of a fan for a while, it will become slightly 'sticky' on the outside. This significantly enhances how much smoke will attach to the meat. Try it!