A Dream Machine
Most of the time, when we wake up from a dream, we forget about it in a split second. But sometimes the dream seems real enough that we have to convince ourselves that it was just a dream. And in some extremely rare cases (at least it happened to me) when we recall a dream after some time, we have to really think hard to recall if it was actually a dream or reality (given that it was not a fictional one).
If we can find a way to create a dream machine that can forcefully put a real enough dream during the victim's sleep, and then in real life, you show them minor evidence to prove that it was real, It will act as a false memory.
The victim might be confused at first but in the long run, it should work, but the minor (but false) evidence will play an important role in it.
How does the machine work?
Some kind of EM waves alter the synapses states or stimulate electrical signals that creates the dream.