I have a stainless steel food service pan that I fill with water, and put that on the very bottom of the main chamber. Then I take the charcoal grate and invert it over the pan of water. Skoootch the inverted charcoal pan all the way to the right, so it just sits on top of the bolts that bolt the firebox to the main chamber.
Keeps the direct fire away from the food that way, and the water helps to regulate the temps in the chamber. I find the temps tend to swing more without the water in the bottom.
Tri Tip is a great piece of meat. I'd never even heard of it until a few weeks ago when browsing this site. Yesterday, I was in a wholesale food store, and they had these 12 to 15 pound cryo packs of Tri Tip. So I picked up a bag, smoked 3 tonight and have 5 more in the freezer now.