Unless there is some compelling reason for such a large opening into this septic tank, I recommend that the opening be reduced with the use of a circular concrete spacer. Convert the rectangular manhole opening to a 24 inch circular opening. Circular manhole covers cannot fall into the tank.
The thickness of the concrete spacer is 2 to 3 inches with an outside diameter of 36 inches, and an access hole diameter of 24 inches. Seal the spacer to tank top, and cover the corners of the rectangular opening with an appropriate epoxy cement material to make the tank watertight.
Fergus Power Pump, Inc. sells a septic tank sealing ring, and riser lid made of plastic. Install the “SEAL-R” ring, a short piece of 24 inch diameter corrugated plastic culvert pipe for the riser, and then install the lightweight, yet strong, plastic lid with the stainless steel screws. Fergus’s webpage address is:
I have performed this very same procedure on several occasions. Good Luck. JWA