If you want to get more from your BBQ, why not turn it into a pizza oven? While there are commercial add-ons that do this, you can make your own version for much less money.
YouTube user madbare explains how to transform cheap steel flashing -- which you can get from a hardware store -- into a clone of the commercial Kettle Pizza. Although you don't see him making the contraption in action, the design is very straightforward. He used a plasma cutter for the hole in the middle, but you can use a jigsaw instead.
In the comments on his video, he says:
Worked great and saw temps at the stone of 600F and in in the dome close to 900F. All in all I say it worked great. I did end up adding a couple more bolts to hold the stone in place and added a few 1/8 sheet metal screws around the top.