Technically, since you're sending the air through the filter multiple times, you don't need a HEPA filter. HEPA filters are optimized for single-pass airflows.

Technically, since you're sending the air through the filter multiple times, you don't need a HEPA filter. HEPA filters are optimized for single-pass airflows. And there are thicker filters which give you more pleats, and thus less air impedance, in addition to the "triangle" method suggested by a sibling comment.