I did 15 pounder recently on my 18.
I had to block up the center with a wood block and foil the ends to make it work.
I used water in a deep dish pizza pan.
The water made the cook on the cooler side of 225 until it evaporated.
I think the temp ran about 280f or so the last 4 hours due to water evaporation.
I started the cook at 20:00. At 02:30 I woke to check on it and it went down to 180f so I raked the coals to increase the heat.
The cook took 16 hours and then was served after a 4 hour rest wrapped in a cooler.
Tasted better than any brisket I’ve ever had but I don’t go to Texas

.
It was so tender that next time I might take it off at 195f and rest it longer.
One thing to note,
I ran a full Arborfab coal basket and only used my front lower vent for the most part.
I was getting sparks landing on my deck so I had to hose off the underneath area of the deck and lay down some sheet metal for fire protection.
For future cooks I will soak the bottom of the deck prior to the cook, have a hose on the ready and have a wind block.
Good luck to you.