Todd:
The bag filter is of type 24X20X14 inch Vee Bags with loops, part Number 242014VBL, about 50$ ea, available only in two-packs.

Al:
My filters are upstream, i.e. on the suction side. I believe that is what everybody does, because it keeps the fan and the blades clean. I agree with you that a somewhat smaller fan than mine would have been better due to size and weight considerations.

Jim:
The quality of your box is really impressive! I believe that axial-flow fans are less capable of overcoming a pressure drop from filters, so you may have less flow than with a comparable radial-flow squirrel-cage fan. Anyway, your air cleaner is surely fine.