bottom intake feeding the GPU is probably the most obvious high-value active airflow position in the conventional layout of the M2 and M3. The missing factory filter is a genuine omission.
you can't even buy a bottom filter from them. theirs says for the front or top. the manual recommends side intake with top and bottom exhaust to minimize dust yet I didn't see any builds on YT doing that. Even their own youtube tutorial doesn't do that.