Given a 24 word BIP39 mnemonic phrase, I would like to have a way to split it into 5 shares, so that any 3 would allow me to reconstruct the phrase (but any less than 3 would not).
This is easy enough to do with Shamir Secret Sharing, and very secure. But that requires, at the minimum, a computer, and I would rather have a way to achieve the same (or something similar) with only a pen and paper (and let's say not more than an hour's work).
Now, I don't mind it being slightly less secure, in the sense that obtaining 2 of the 5 shares makes it easier to guess any of the other 3, as long as the difficulty is reasonable (e.g. 90 bits entropy). I feel like a 24 word phrase is really entropy overkill, and a third of that entropy is, by itself, quite a lot and sufficient for my purposes.