As others have said, you can just create an offline/cold wallet either using software (like Electrum) or a signing device of some kind. But it's worth noting that 18 years from now, Bitcoin may look quite different. While a wallet from 15 years ago will still work, it is much more expensive to send transactions with it. It's possible that in 18 years, Bitcoin will have at least partially or maybe even fully moved to quantum-proof signing algorithms. If quantum computing becomes a reliable attack vector in that time period, old wallets may be useless.
Seed phrases are great, but will the software used to generate that seed phrase still exist in 18 years and be easy to install on the latest operating system? At least with a simple private key, it can be universally imported to any Bitcoin wallet, whereas a seed phrase requires knowing the derivation path and having software to support it.
So my answer is:
1. Make an offline/cold wallet using any wallet software, electrum is well known and supported
2. Receive all the funds you wish to transfer into a single UTXO
3. Export the private key for that UTXO.
4. Put the seed phrase AND private key onto metal or archival quality paper
Another consideration on that timescale is natural disasters. What if the house they store this thing in burns down or is burglarized? Using a multi-sig wallet can help with this, but will make things more complicated.