法国采购需求：螺母，铆螺母

I'm looking to purchase your countersunk hex rivet nuts and I have a specific requirement: I need the surface to be completely flat after installation. My challenge is figuring out how to create a countersink on a hex hole in the sheet metal to ensure the rivet nut doesn't protrude more than 0.5 mm. Could you provide some guidance or recommendations on how to achieve this? Your expertise would be greatly appreciated as I want to ensure a smooth finish. Looking forward to your advice!