In that case, I would suggest a temporary brain wallet is most secure and least likely to get you in trouble with customs officials.  If you were to start early enough and give yourself time, I'm sure you could remember a 12-word seed phrase.  You could also add a 13th word that you already remember such as the middle name of distant ancestor, or a secure password that you already remember.

Create a phrase offline and it's corresponding watch-only wallet.  Write down the phrase and store it somewhere safe.  Rehearse it silently for a couple of days, only pulling it out of storage when you need a refresher.  After a couple of days restore it offline and compare to the watch only wallet.  Keep that up for a couple of weeks, after which you'll have it memorized with a significant amount of confidence.  Keep up the silent rehearsals and offline restoration checks leading up to your travel date.  Once you're 100% confident in your memorization of the phrase, transfer your coins.  Once you reach your destination you can create a more secure storage scheme, or write down the phrase that you've memorized in case your memory fails in the future.

Most of us have memorized passages from religious texts, prayers, lyrics of our favorite songs, or lines from our favorite movies.  And if it's only for short period of time it shouldn't be difficult to keep it fresh in your memory.  It's more difficult to try to remember something like a seed phrase for years, but for a few days or a week while traveling, a brain wallet is probably the most secure and safe method. 

Certainly safer and more comfortable than using your prison wallet.