Here's a simple solution if you have one of these sorts of gas grills: has to have at least 3 burners. Because of the lack of insulation in the lid, I only do pizza here in the hotter months of the year where I live--plus then the house doesn't heat up ;o)
Simply drill four holes in the lid, from which to string two 1/4" stainless steel threaded stock across to suspend your TOP stone. I am using a custom-cut stone that occupies most of the surface area where the lid comes down to the BBQ lower case where the burners live. I have a 3/4" 16" round pizza stone that sits on the actual grill. Because heat from the burners flows around the 16" stone and hits the 1/2" flat custom rectangular stone just 4" above the top of the round pizza baking stone, it's possible with a little effort (mainly a 16" aluminum thin sheet sits under the 16" baking stone to slow down heat gain in the lower stone) to get the temp above the pizza a little above (30-40 degrees which is OK) the actual lower baking stone. I cook when the lower stone hits about 700 degrees. Easy to negotiate too because as you lift the BBQ lid up, up goes the upper stone, suspended loosely but securely with the stainless rods.
This is a cheap and effective solution, as long as you're using the BBQ for other things too