It is a binary choice, either you call the left shape bouba and the right shape kiki, or you call the left shape kiki and the right shape bouba, you can't use any other name.
The interesting part is that everyone gives the same answer, regardless of culture and language.
From the article: Without being told which shape was which,
> 95% to 98% selected the curvy shape as "bouba" and the jagged one as "kiki", suggesting that the human brain somehow attaches abstract meanings to the shapes and sounds in a consistent way
So apart from visually impaired (from birth) people, we may have a pattern in mapping shapes to phonemes.
I'm guessing because you misunderstood the question. The question in the research was something like, "Which one of these shapes is called 'bouba' and which is called 'kiki'?" There was no option to invent your own word.
The interesting part is that everyone gives the same answer, regardless of culture and language.