That was my next step, look at the pressure ratings for mm H2O of your fans. Problem with most cases is, they are not a single tunnel like most NAS, but would h

That was my next step, look at the pressure ratings for mm H2O of your fans. Problem with most cases is, they are not a single tunnel like most NAS, but would have multiple openings that might change airflow direction if you add a filter in front of a single intake. For the NAS I trialed a range of Noctuas, but the stock fans (KM121225LL 2x120mm 2.3mmH2O) proved most reliable with the default Synology control settings. So anything in that 2mm Range should work.