MX40-STD1-103
Airguard MX40-STD1-103 Box of 12 20x20x1 Pleated Air Filters FNFP
I picked the 791909-1 because it seemed built to last through daily wear.
Airguard MX40-STD1-103 Box of 12 20x20x1 Pleated Air Filters 3255
A nice touch on the ULTRA2 is the soft-touch surface on frequently handled areas.