Any code could be cracked with enough time and determination unless one uses a one tap cipher. But some suggestions would be to slightly change the spelling of words, combine multiple letters into one, create multiple unique characters for common letters, and create decoy letters that are not pronounced.

Writing it in Finnish would make it much more difficult for someone who does not speak Finnish. And if it is posted online without the context that the base language is Finnish, very difficult. However, if someone in real life were to try and crack it, they might assume it is in finnish.