Straiter, insist that the septic system installer place a proper, gas tight cover on the riser of your septic tank. Not only is the odor obnoxious, but open access to the septic tank poses a public health threat by insects, and other disease vectors, and a potential safety hazard if someone should fall into the tank. Be sure that the lid is gas tight, insect proof, and lockable for safety.
This situation is very dangerous, so have it resolved immediately. It also reminds me of my favorite limerick. :>)
I once knew a man named McBride
he fell in a two-holer, and died
along came his brother
and fell in the other
now they’re interred, side by side.
JWA