I've since had a chance to look inside some commercial units, and I think I've cracked the code as to why the V1 design has less airflow than I'd expect. It seems the trick to getting a high volume/low pressure flow is making the surface area of the filter as large as possible. On paper I began with a box design but I found it kinda tedious to design, especially around different fans/filters and the ability to remove the filter while remaining relatively airtight.

The solution, I think, is to use a filter like those seen in old car air intakes: a long/wide cylinder with the filter media wrapped around it. This will hopefully allow for much higher airflow while still keeping the same form factor and being easily constructed from plumbing pipe.

Hopefully will be able to update again soon, I'm re-orienting the lab which gives me a good opportunity to uninstall V1 and work on it.