Vincent Bertone, if the “septic cleanout” is merely the cleanout for the sewer pipe between the house and the septic tank, then a “large hatch” would be an acceptable solution to your dilemma. However, if the “septic cleanout” is the access manhole into the septic tank, then I recommend that you install a new septic tank, and abandon and remove the old septic tank. I do not recommend that you build a deck over a septic tank.