I don't think there is a technological solution for this unless you have some sort of sleight-of-hand. Typically, border agents of countries with lots of transit do not stop people for very long. Some other countries (North Korea, perhaps) might put everyone through the wringer because they do not have a lot of crossings. If a border agent of a relatively free country is stopping you, they probably have some suspicion, in which case it is best to not be holding evidence in your hand.
There are steganographic methods to hide your stuff. You can also use burners on either side of the border crossing and keep your main line clean. But bringing a device full of encrypted data (even if it's just your regular photo collection) that you refuse to unlock will probably be suspicious.
I know that there are times when there are no reasons for suspicion and people get stopped anyway. The border agent didn't like your look, or racism, or an order came down from on high to stop everyone from a particular country and annoy them. If that's the case, it's probably still best to not have a lot of incriminating evidence on your person, encrypted or not.