The filters will get coated with paint very quickly and require replacement. Consider cheap glass filters and cover them with disposable non woven fabric. A little tape will old the disposable fabric in place til fan suction nails it to the filters.
My experience says you don't need a "wind tunnel". Go easy on the fan.
RV-4 fastback in process w/ Superior roller 360/AFP/G3X/CPI/Catto3b