I’ve actually built this fan “mod” and when I change out the filter, I’m going to try attaching it to the back instead.

With the filter thoroughly taped to the front (nearly airtight), there’s a large amount of blowback. I suspect that this is reducing the overall efficiency of air filtration.

With it taped to the back, you don't get blowback, but there is significantly reduced airflow out the front. I used a filter of this type for ~2 weeks during the Thomas fire. Worked very well, though the proper filter I received from Amazon a week after I built this worked better. Two is better than one, though!