Put down some inches of Crusher Run/Quarry Process. Put in some wood racks like this https://www.amazon.com/gp/product/B000ZHQC82/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1
Or build something of your own in the space. I own 8 of those types of racks. All full at the moment. Space the wood apart at least 6 inches between each row for air circulation. The wood must be off the stone or the bottom row will never dry properly and you’ll introduce stone into your burner.
I’ve used that type of rack. Darn good, durable, flexible and simple. Hard to beat that combination.
i have two of those indoors. when we cut, split and stack, the 12x24 lean to is stacked to the ceiling and completely stuffed to the brim mostly with harvested wood (we live near a swamp) so swamp maple and swamp willow