with LUKS you can just use a detached header stored on a USB thumb drive. Then just toss the thumb drive.
But this isn't plausible deniability. You still have a hard drive in your possession and it's still covered by random data. Better deniability: always use an external drive and then distance yourself from the drive. You want plausible deniability of the entire drive to the extent that no one even suspects you of being the owner. Or, have such little sensitive data that you can use steganography to hide it in an image or video file. Just don't put the steganography tools on the same computer as the hidden data.
But this isn't plausible deniability. You still have a hard drive in your possession and it's still covered by random data. Better deniability: always use an external drive and then distance yourself from the drive. You want plausible deniability of the entire drive to the extent that no one even suspects you of being the owner. Or, have such little sensitive data that you can use steganography to hide it in an image or video file. Just don't put the steganography tools on the same computer as the hidden data.
Anything else is pretty much a joke.