Sand will give you the most stable temps.
Water a close second if you keep the pan filled. That's what started this whole debate. I put on a brisket one morning with water in the pan. This was around ten years ago. I then got called into work. When I got home that night I had a charred brisket. So I started looking for an better way to keep the temps stable. I tried epoxying two pans together with an air gap, fire bricks in the pan, crushed rock, and sand. The sand seemed to work the best because it didn't punch holes in the foil and let the grease through into the pan. I used sand for a few years but not the way most people do. I put just enough sand in the pan to be able to push it up the sides of the bowl, then held it in place with foil. I wanted about an inch thick layer, not a full pan. I was thinking, heat deflection, not heat sink.
Later, after experimenting with layers of foil in an empty pan, I continued on with this method for many years. Foil the pan, add a second layer but leave an air gap, add a third layer on top with another air gap. I just lay the foil down and crimp it around the edges to hold it in place. I just wanted food on the bottom rack not to burn.
Now, I had meant for this dry method to be a competition secret, but one of my knuckleheaded friends posted the idea on another bbq forum. and the idea spread. The next bbq season, I drove down from Kansas City to Louisiana for a contest and the Texan next to me was setting up some bullets and he pulled out some pretty, white sand out of a container and filled his water pans with it. I had to laugh when he said he'd read about it on the bbq forum. Since then lots of the great cookers on this forum have come up with some excellent ideas. Two pans bolted together, flower pots, etc. I'll bet they all work great. So, try it both ways or come up with something different. That's what's fun. It's the journey, not the destination.
