A steel holding tank? Use a non metal specifically designed sewage compatible product whether FRP, HDPE, PE etc.
That said, for a mild steel tank, construct it so top is flanged, bolted and uses a good rubber gasket. The purpose of the removable top is to provide perfect accesss for metal prep and application of a spray in two or three part DuPont coating specific for sewage work. For smaller jobs hose free specialty small spray auto shop containers are available. The nipples and fittings will deserve special attention for proper prep. Can't overemphasize adherence to metal prep protocols.
I've seen on small tanks where SS is cheaper than a specially coated mild steel tank.
Every foreign country I've worked in seems to have a decent FRP fabricator, have you pursued this route? Shipping in a non metallic tank from Italy or Turkey could be done too.