Just for our metrically challenged friends...... 600mmx 400 mm =23.62 x 15.75 inches ( a very very very short skinny woman ) In one piece, of course. if not...well....I live locally , about 18 Km from Mt Clear in a 4 bedroom home on acres … on septic,and I think my set up ( now 40 years old) would be fairly common.
Most are set only inches below ground level , the curved concrete lid measures 600mm x 400 mm( there no doubt is an inside contact perimeter shoulder of perhaps 25mm ), so I’m thinking an intact female body could be pushed inside.
Most installations would not be as readily findable as mine!
Can I ask what tools you use to open yours up, Dr Samoht? Unless you tear it off with your bare hands. not that there is anything wrong in that, but just out of curiousity...