I had also thought of taking the seeds written in a book or notebook on several pages, but with invisible ink, so that no way that it will occur to customs agents to search through a book of 300 pages.
You can always use an extra word (the passphrase) with your seed phrase for extra security. Something you can remember. It is easier to remember 1 word.
You can also always write down numbers instead of words. Just go to the wordlist page[1] and write down the corresponding number for each word in correct order. For example if the first word is "leaf" and second word is "alien" write down 1014, 51 and so on. Make sure to test what you wrote by recovering your wallet to make sure there is no mistake.
https://github.com/bitcoin/bips/blob/master/bip-0039/english.txt