Pizza Stone cut to fit into a toaster oven
I expect that will be a problem because it will introduce a lot of thermal mass, and hence inertia. The problem that cheapie toaster ovens have is heating up and cooling down fast enough to track the recommended reflow profile: the profiles I've seen are (from memory) around 2C / second, and my oven can't do that. Then in the cooldown stage you need the oven to dump heat quickly, and the pizza stone will tend to keep it hot.
Rather than a pizza stone I think you'd have more success with a simple sheet of metal, which would have smaller mass and conduct the heat quickly across its surface.
Jon