Product review (5 stars): Aprilaire 35 Filters are the only replacement option!
These are the only panels to purchase when replacing the original pane for a Aprilaire humidifier system.  My OEM panel finally started plugging up and was leaking water onto the basement floor, which is a good indicator that it required changing.  I replaced with the OEM Aprilaire 35 filter and was back up and running in no time with no leaks.  Do not purchase another off-brand product since they will not be the metal mesh-style filter.  The 2-pack is also great value since you'll have another one on hand for the second heating season.