Hi Dave and welcome to the forum.
What kind of loads are you anticipating on the cover? Is it going to be a point load like a rear wheel of a tractor, or is the load going to be distributed evenly over the entire areas as in dirt and grass? Are there going to be holes in the cover?
Using 1" steel plate is going to be the most expensive option compared to a welded mechanical truss approach and maybe 1/8" or 1/4" plate skin depending on loads.
You can get varying toughness grades of steel, but not really hardened. Generally the tougher grades are used or damage resistance rather than integral strength so probably a general use Mild Steel sheet would suit the purpose. Before we go calculating the thickness required, we need to know a lot more about what is, or might happen with loads.
If there is a risk to humans passing over it or under it, then it is doubtful you will get a lot of assistance here as we would recommend a Structural Engineer be employed as there will local Codes to be met.
Just did a quick check and a 1" x 4' x 8' hot rolled steel plate runs to about $1800 and you will need three of those as I am not sure you will easily find an 8' x 12' in one piece.