Custom topography extents

I have a Leapfrog Geothermal model with the standard XYZ extents that make a nice square, but am I able to customize the boundaries? My advisor wants it to look like this (picture attached) and suggested I "cut it to this shape" but I'm not sure how to make the boundaries diagonal.  The picture is looking straight down and the current boundary is just on the  XY plane.  But I need the boundaries to be the black lines.  Thanks for any help!