No need to rent a saw to cut completely through slab. Install masonary blade set to cut 1"to 2" in your hand held power saw. Save's wear on blade,cut's faster and eliminate's cloud of dust if you trickle water on blade while cutting. Square up edges with cut then chip off with chisel. At that point,you can repour/paver original section or simply set form against 2 rough edges and fill void. Hope that make's sense.
Gonna require special provisions to support wheeled traffic. Even if riser and lid are indestructible,weight of vehicle will transfer to septic tank lid.