How did you facet/cab this piece? I'm thinking it wasn't dopped...
I started by making a square preform from a lortone template on an 8mm thick slab, I marked the square out with a brass rod sharpened to a very fine point so the line is super thin, the thin line really helps to be as accurate as possible with regards to the shape. Wide lines are comparatively easy to grind through part of the line without noticing.
I always use a cab rest when grinding out the initial shape so that the angle of each side is exactly the same.
Once I have my shape I draw two super thin lines around the sides of the preform to mark out the girdle. I do this by putting the top of the cab face down in a super flat surface, and the brass rod lays flat on the same surface with a shim underneath as necessary to get the two lines at the desired height. Pin down the brass rod with one hand so that it can’t move up, down, or side to side. Then I rotate the cab while pressing it into the rod to get the line marked on the sides, shim up the rod and repeat. Be sure to sharpen the brass rod as necessary to make sure your lines are super thin for the same reason that you don’t want to grind through part of the line without noticing. Make sure the same face is down while marking both lines, this ensures that they are parallel to the face and each other, even a slight variation in slab thickness will mess that up if you have the bottom of the cab face down on the table.
Draw an x with a straight edge on the back from corner to corner as a visual guide for cutting the four sided back, again super thin lines are the ticket to maintain accuracy. Doing this before cutting the dome is a lot easier.
Dop right onto the x and proceed to cut/polish the dome down to your first girdle line as you would any other cab, but leave the sides alone so you don’t remove your bottom/second girdle line.
Remove the stone from the dop and flip it over then dop the finished top of the cab so you now have the x on top. This is where I need to show you with pictures on how to cut the four sided back because it’s pretty hard to explain exactly how I do that, but basically the four sides get cut on the hard wheels and once you have the shape proceed to soft wheels and incorporate sanding/polishing the girdle along with the four sides at that point.
Really it’s not all that hard, just a lengthy process comparatively to cutting a standard cab.