Earlier quoted context omitted.
I suggest installing a shell OS (with a neat GUI, no hacker-ish looks) on the Notebook's first partition, the rest (where the main Linux installation is residing) will be encrypted and can be decrypted and booted into only using a USB bootloader with the missing LUKS header (which can be downloaded later using a VPN, for example).
If they ask you "is there any hidden data on this device", you would have to respond in the affirmative. Lying to a border guard is a felony, there's no cute technical solution to this. They will, according to their own paperwork, make copies of any data they find interesting for future analysis.
Disclaimer: this is all being speculative, of course. I'm not suggesting to lie to or misdirect a border guard in a real-life situation.