I was able to trade into a set of Grillgrates for the Weber kettle, and using those on top of a spare grate seems to do the trick. I think that any piece of metal (or whatever) will work, so long as it creates a ceiling over the stone and reaches the back of the grill (so the flames will roll forward). In a perfect world, it has a rounded side that can contour the back of the kettle. The more it can retain heat and radiate it, the better.
Only caution with using a secondary stone is that it better be a brute. I've cracked two stones when I get to high temp (the "light duty" ones we had lying around for regular oven use... not heavy/thick ones).
Note: I also checked with a nearby metal fab shop. They quoted me $60 for a plain steel 3/16th circle at 21" diameter with 2 small cuts on opposing sides. Would have been plain steel (and needed seasoning or care)... but way cheaper. That was what I planned to do before I traded into the grillgrates.