We've never needed one.
I put the reversable griddle on first (flat side up) then the bbq grill thingy on top of that, whack the pizza on, stick the lid on and leave it on 1/3 power for around 10-15 minutes (keep a check on it of course).
It doesnt melt the top very well, but cooks the rest of it great. We also do garlic bread etc this way as well.
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