Pizza Stone Size

I've read that you experienced cooks recommend using a pizza stone or ceramic tile to even the heat within the oven. I am assuming the stone would sit directly on the bottom metal divider, which is about 4" above the bottom of the oven.

I would like to do this mod, but what size?

My oven is 15 1/8 Wide (13 1/2" between the holes) x 13 1/2" deep (12 1/2" from back to the holes in the metal divider.)

Can you cover the holes?

Suggestions and comments appreciated.

Thanks.