Short of handmade you have two options. Use a made mount either the
next size up or down, 10x8 or 9x7. That’s a deep stone so maybe its
got a bulging belly too. In a case like that I’d rather use an
oversize. If the underbezel shows obnoxiously you can bevel it so as
not to cast a sharp reflection straight up, a knife edge basically.
Or maybe cut the setting down as needed.
If you opt for undersize, sometimes you can stretch a setting.
Either jam a suitably shaped punch (modify an old dapping punch, or
use expanding pliers, or squish the top rail wires with half
round/flat pliers, lengthening them. To make the length/width ratio
better, just squeeze the sides in with wide flat pliers, this sounds
like a longish oval stone.
Of course, its much easier if you start with a semi that has no
center, modify as above then solder it in.