An idea to achieve tamper evidence and protection (though we had the discussion if it's even a good idea or not): weld on the nut.
What about rust? I know that's usually not a problem with washers I've had laying around forever, but if it holds crucial information, I wouldn't mind oiling them before bolting them together. Especially since this won't be in a normal drawer but probably somewhere well hidden.Quote
Protection: Not everyone just has an angle grinder.
I can just imagine myself getting my angle grinder to buy something 
I don't really see another way to hold a round object in place other than friction to be honest.
Have you considered a magnet? They're available in many sizes and shapes on AliExpress and can probably fit somewhere inside the 3D design. Unless you're going for stainless steel washers, but I assume those are too hard to stamp.