My house has 2 septic tanks the one not being use is cracked on top about a 3'x3' section. I was thinking of putting a steel plate over it or could I fabricate a concrete cover 4'x4' cover w rebar myself? Plow truck will go over corner of it and I don't want the expense of filling it w dirt. Any suggestions?

Dirt's cheaper than concrete but you can pour a lid if you want. Use 5/8" rebar 12" O.C. , and fiber reinforced concrete.

I did a 3 x 5 foot concrete pad that was 4 inches thick. I used rebar and 20 in pvc pipe 3ft high with matching cover. Used 3/4 inch plywood for base and 2 X 4 for the frame. I also installed a couple of 1/2 inch "U" clamps for lifting the pad after it was properly cured. Metal will rot quickly by the acids in the waste tank. Make sure the form is flat for drying. You may wish to oil the insides of your frame to prevent the concrete from sticking.

Good Luck
Best Wishes