Those are crush washers: the edge is small, and the hole is big. I've never seen a "normal" washer made out of copper, it's too soft to be used as construction material. So there's not much space for stamping words.
Maybe, I never saw exact size of this washers.
I don't know how soft copper is exactly, but it was used for centuries as a bronze for making swords, axes and knifes, especially during the bronze age.
If you scratch the ground in Europe you could find a bunch of copper objects, museums are full of it, and bronze is alloy of copper and other ingredients to make it harder.
When you consider price, I mean the loss of strength in copper probably isn't worth it. I think copper should be left to pipes in your home, and not as a way of storing something important on them. Stainless steel, and potentially titanium are really really the only options you need.
Someone else mentioned copper first, and I know it's not perfect but it's still much better than aluminum and other cheap metals.
If you already have something copper at home than it's possible to use that as custom backup, nobody will think anything suspicious about it, even if it's pipes we are talking
Here is Jameson Lopp testing Copper Safe Seed plate that passed all tests with perfect score, and it costs only $28:
https://jlopp.github.io/metal-bitcoin-storage-reviews/reviews/safe-seed/I still have to figure out a single advantage of copper instead of stainless steel.
Let’s try to stick to useful ideas, not speculations about every possible permutation.
Copper exists much longer than stainless steel, so it's funny to say that this is some speculation...
If you already have copper you can use it, that is an advantage for some people.
. I'm not sure how much pipes or copper cabling weighs mind you, might take a bit to get to the price per kilogram. 
