I figured out a pretty cool method for inlay years ago. It's tried & true...never lost a stone setting. Here's how I do it:
*trace the outline of the stone onto the mask (has to be a flat-backed cabochon or it doesn't work, by the way)
*draw a line inside the outline, about 1/8 inch smaller.
*cut out smaller hole.
*smooth edges with skiver
*put stone in from the back. It won't fal through 'cause the hole is smaller than the circumference of the stone.
*lay thick suede (or 2oz split) over back of cabochon on inside of mask...be sure it is at least 1/2 inch bigger than circumference of cab.
*use barge cement (dry mount) to sandwich the stone in place; suede patch, cab, hole.
You end up with a peekaboo stone setting that'll never fall off. ^_^