I tend to use a cheapo pizza pan for a diffuser. I started without one and had problems with meats in the center and bottom rack crisping up more than the outer edges. I ended up with the diffuser, and after a few cooks decided to drill a bunch of 5/8" holes in it. Problem was when I cooked spares once, when I opened the lid to check the meat the rendering fat on the diffuser lit up and I had a massive flame. Since then I have always used foil to wrap the diffuser, makes for easy cleaning too.
I have done a few cooks without the diffuser, but prefer it. Now, when I do chicken, I start with the diffuser in place, then when ready to finish the skin I remove the diffuser in order for a more direct heat to crisp it up a little. works great!