I have a cabin in north Georgia. 700 square feet, basically sitting half on block foundation and other half exposted to ground.

I have since had the exposed area dug out and retaining wall put across the edge of the block foundation. My question is I want to add a new room below this area.Other cabins have just built a regular deck flooring then finished the room leaving underneath still open for who knows what to get underneath.It would be difficult for me to do concrete,and if I use joist and plywood, what can I safely use to keep moisture and critters from getting underneath. It will only be about 4 inches of the ground.I know the usual plastic as vapor but covering the sides without venting is my concern since its not concrete.thanks