Hi Gustav just listened to your piece! I'm not really into atonal sounding pieces (except if it used as an effect) so i will not comment on the piece itself.
I just want to react to what you wrote in the description to clarify the method i described before:
Yeah that's true, however while the aim with this method that every line on one hand should be independent and should stand on its own as good as possible on the other hand it should sound good together also (one should be able to synchronize their vertical and horizontal writing and balance them). Your free lines are your horizontal lines but the resulting chords are your vertical structure and one should keep attention to this also how you leading voices or how your chord progression sounds.
It's not an easy task i know i try to learn this as well, just wanted to point out that if you were to aim for a atonal sound that's perfectly fine, but one can write 'diatonic sounding pieces' with this technique also or write in any style really. So don't think about it as mixing modes randomly to generate random results. One can pick scales that are close to each other also, point is to have a logic in your mind and aim and to learn not only to start writing music by writing block chords first (vertical structure), but one can start with free lines (melody and countermelodies, free lines etc- your horizontal structure) as well. If one are able to balance these two really good results can be achieved.