You can put an oven safe bowl under the brisket, to get it to fit better, until it starts to shrink up. I have done this before with great results. I used a CB Stacker on my 18 Outrider.
This a full packer ready to go on the Outrider...
I only used the bowl to help pull the edges of the brisket away from the direct heat rising up the sides of the stacker. Once it shrunk up, I removed the bowl and kept cookin. If there is no fire directly under the brisket, I don't think you have anything to worry about. This is a full packer on a 22 kettle, using a standard snake. Plenty of room...