Inverting shapes

Hello everyone,

I am trying to get the inverted shape of this crater like surface precisely as possible.
As you can see in the picture (circled by a blue line) there is the surface I need to work on.
In the other pictrure you can see the inverted shape I have made - only I need it to have a baseline which is not a perfect circle or ellipse but the exact border base line the original crater has.
I have manged to make this inverted shape by creating a cylinder form and then use the meshbooleandifference command.

Would really like to know how I can get a better outcome of the inverted shape and the most important - with the exact base line the original crater has.
thanks a lot in advance.

Hello- if the exact border shape is that rectangle, and those are mesh edges, then Explode should get you that area - but I am guessing - feel free to post the file or send to with a link back here in your comments.