Here is a kit that turns your gas grill into a pizza cooking chamber. The KettlePizza Gas Pro Pizza Oven Kit has a stainless steel construction and ships with a 15-inch cordierite stone. It works on grills with at least 3 burners. To get started, put the enclosure and stone on your grill, let it heat up for 20 minutes and you are ready to go.
This pizza oven insert weighs 16lbs and has a convenient carrying handle. More info is available here.