How Do I Cut Out A Rut For Crystals?

[TerribleOne]

I'm not sure how to title it really but I want to know how to cut out a depth along a collar so my crystals can sit down in there snug and not sticking out on top. Here is exactly what I'm referring to in the picture.

Is there a tool? A stamp to impress deep? Razor? I don't know how to get that good clean deep rectangle

[Tree Reaper]

That would be an overlay.

The crystals are put on the strap and then another piece of leather that has the center cut out is sewn over the leather that holds the crystals.

[TerribleOne]

Thanks.