I know the cost section might put a lot of users off the idea of doing this, but ignore the total cost, because the total cost is likely not going to to be anywhere near that. Since, most of these items, people already have especially if they're a home owner. For example, I have millions of washers/screws (including the size required), I have a sledge, gloves, duct tape, hot glue gun, and a micro fibre cloth. That has already reduced the price by 35+ euros based on the OP's pricing. Although, I bet I have a tonne of other stuff in storage that can either be substituted to a degree or already have. For example, the stickers could potentially be substituted.
Honestly, the raw material cost is nothing. 4€ for washers and a few euros for the stamp (you can skip that if in a pinch). Tools should be easily available in any household or can be borrowed by some friend without any weird questions.
All the other stuff like tube, stickers, hot glue etc. aren't needed, so while fillippone went total overkill, I think washers are actually the absolute cheapest way to get a steel wallet backup.
The letter stamps have to be purchased with any type of steel wallet product, except those where you slide the letters, but those aren't recommended since they fall out if the casing bends (e.g. under rubble). A stack of washers (especially larger ones) is extremely solid. I am sure you can even find it in a destroyed house and restore the backup.