https://www.titane-services.eu/epages/230155.sf/en_GB/?ObjectPath=/Shops/230155/Products/TIR-MU05GR5 ~1.50€ each
Since you can do a 24-word backup on 12 washers, plus one to hide the first word when everything's screwed together, it will be between 20 and 40 bucks. Just in case anyone absolutely wants to do it for whatever reason..
https://www.pro-bolt.com/titanium-flat-washer-m5-15mm-od/ - in this link there are washers with an outer diameter of 15mm, which I consider very small and it will be problematic to put text on them. Also, the thickness of the washer seems to me too small - only 1mm. As a result, the dimensions and thickness of this particular sample will not allow you to write 2 words for each washer. With due skill and necessity, this is doable (despite the miniaturization of the text), but it is better to choose https://www.pro-bolt.com/titanium-washer-m12-28mm-od-form-c/ - washers with an outer diameter of 28mm and a thickness of 2.5mm at a price of 6.6£ each.
Let's calculate the costs:
12 words + 1 hiding the first word = 7 washers (if applied on 2 sides) = ~46£.
24 words + 1 hiding the first word = 13 washers (if applied on 2 sides) ~ 86£.
Multiply the price by 2 if only 1 word will be writed on each washer.