It was my understanding that 5-th amendment protections against unreasonable searches are suspended at the border. That would certainly include customs at the airport. So, in a strict legal sense, demanding to unlock a device does not violate the 5th.
https://www.aclunc.org/docs/ACLU-NC_2019-03-28_Letter_re._El...
> CBP must ensure that its officers comply with the U.S. Constitution. Even at the border, the search of an electronic device is governed by the Fourth Amendment. To satisfy Ninth Circuit and Supreme Court law concerning electronic searches, any such search should be based on a warrant and be limited in scope to information relevant to the agency’s legitimate purpose in conducting the search. The attempted unconstitutional search of Dr. Gal’s devices illustrates that CBP’s policies do not in fact include the requirements necessary to safeguard the constitutional rights of people at the border