... OK, the DIY project looks like it might happen.
Please let me know if you think this will work.
Here is a pic of the KettlePizza gas grill "pizza oven" accesory:
It isn't obvious from the still pic, but the back side is folded over, just like the sides.
Here is a video that lets you see how the thing works, and also shows the back side.
As I watched, I realized that this is nothing more than a big baking pan with one side cut out.
So, why not buy a cheap baking pan and cut out one side??
The KettlePizza gas pan's dimensions are 17 x 24 x 3.5 inches. It is made of stainless steel which, of course, is an expensive material. In thinking about a cheaper material that would also work, what about simple steel, like the steel frying pans that so many people love?
Here is a steel roasting pan that is 18 X 24 X 3.5 inches, exactly the same size as the KettlePizza gas grill insert:
Johnson-Rose Roasting Pan
As you can see if you click on that link, it is $55, with free shipping. I could almost buy five of these for what one KettlePizza insert costs.
So my plan is to buy this roasting pan and then simply cut the two front corners, from the top down to the bottom of the pan. Once cut, I'll then fold up the now-free edge, dress the cut edges to make it safe, and then start making pizza.
The steel roasting pan material is dark instead of shiny, but I think most of the "magic" in the pizza oven kit comes from making the heated space really small, so the space immediately around the pizza stays super hot. Any radiation from the metal down towards the top of the pizza should be mostly radiant heat and therefore the roasting pan's black surface might even be a little better than a stainless surface (which will probably go black after a few uses).
I'd be interested in what other people think. If I get enough positive input, I might just go ahead and to this. Two small cuts with a saber saw and then a little touch up with the Dremel tool is all it looks like it would take. I'll have to decide whether it is worth sawing out an oval hole for a handle, to match what the KettlePizza people did.