I did just that with my 2-burner, Force-10 oven. I used a standard "pizza stone", except in my case it was grilling stone used on a fondue set. It was the perfect rectangular size.
I think I put it on the flat metal heat disperser, or maybe right on the bottom beside the burner. Can't recall
exactly. What I can say is that I tried it for a few months, then tossed it over the side. My observation was that it didn't functionally help.
Yes, it retained heat, but it also meant the oven took a lot longer to actually heat up. Makes sense since it now had to heat the heavy stone.
My sense is that these ovens are so small, that they heat up quite quickly, so can be re-heated if/when you open the door. Again, my theory is that the "pizza stone" idea makes sense for larger ovens, but when we're talking something not much bigger than a toaster-oven, it seems counterproductive.
Just so you know, I use our oven a lot when on board. We bake all manner of meals
, including bread and cookies and other deserts. We use it a lot.