The oven floor is supposed be painted black, but I never took the time to do that. If I ever do, I am supposed to use flat black barbecue paint.
Black is best because it absorbs sunshine. Shiny black reflects light. Flat black absorbs light. BBQ paint is meant for high temperatures. It is not likely to off-gas something toxic into your food. I am not certain of this, but this is likely to be food-safer than regular paint.
I always use the baking stone and sometimes I stack two of them in the oven. I haven’t tested whether two are better than one for heat retention. The regular instructions for box cookers do not say a baking stone is necessary. I think it is necessary because I don’t use a reflector. The baking stone works like a heat sink. It retains heat and helps the cooker keep a steady temperature when sunshine is blocked by clouds temporarily.
One advantage of my lazy version of an oven floor is that the cardboard can soak up a spills and be replaced like a paper plate. Of course having to make another oven floor would be too much work for me so I am very, very careful to not overfill my cookware.