I have always used just the beer cans too - no rack at all. HOWEVER, it's not a bad idea as sometimes the edge of the beer can be in a crack and a couple have tipped over - very sad when that happens! lol
We make these ALL the time!!! It is the best chicken you will EVER have! It takes about 2 hours on the grill (coals all around but not directly under chicken) or 4-5 hours in a smoker. The last time I did them I used the charcoal grill, put an aluminum throw-away pan in the center filled with apple juice. Soaked some Hickory wood chunks. Lit the charcoal, threw in the wood, placed the chickens above the aluminum pan and they are wonderful. Also, the bit of skin at the top I make sure covers the hold and I toothpick it so it will stay covered.
A friend did some this weekend but she used a chicken holder with no beer or liquid that steams up in the chicken - they were NOT as good and turned out a little dry (the white meat anyway).
Beer Butt Chicken W/Mexican Flavor
For just a regular rub you can use roughly equal parts of brown sugar and kosher salt, (1 TBS each), then about 1 tsp. each of dried oregano, paprika, garlic powder, 1/4 tsp. cayenne.
Or find a chicken rub on the net.
I have done these in the oven (in a cake pan so the grease has somewhere to go or, on the grill as described above). You will amazed at how tender these are.