Mr. Squid-

McMaster has some washers for socket head cap screws, these are generally made to fit into a counterbore for a cap screw, (these are sometimes referred to as "hi collar" lock washers). Their stk #91111A124 is a stainless steel split lock washer for a M5 screw, the washer ID = 5.4mm, the washer OD = 8.8mm, and the thickness = 1.5mm. A standard M5 socket head screw should have a head diameter of 8.5mm so this washer is .3mm larger than the head, hopefully the counter bore for screw has that much clearance. by the way a package of 100 pcs of the aforementioned washers will set you back $2.90.

take care
bernie