We bought some 1/2 inch insulation, cut it to size and laid it out.
After the insulation was down we bought some 1/2 inch plywood and laid that down.
As plywood will do it was bowing and curling up at the edges which was no good.
To fix the bowing issue we bought some flat head screws and counter sunk the head flush with the wood. We did this on each corner and in the middle.
Worried about putting holes in the floor we used rubber backed washers and spray painted the bots with Rustoleum in hopes to protect it from rust.
Since we had some left over plywood we made some trim to go around the edges. We used wood to metal self tapping screws to secure it. We then laid a bead of sealant on the cracks. It helped keep the wood nice and secure along with the floor bolts.