Magic-Pak Filter

Rentals we acquired have ~10-yr-old Armstrong Magic-Pak HWC Thru-the Wall heating/aircon. None of them have filters anymore! After some experimentation with standard cardboard-frame filters found at ACE (which didn't fit the filter holders) we found a manual which indicates the original came with a "permanent-type" filter, which from pictures had no frame.
Where could I find a suitable replacement? Would a standard cut-to-fit AC filter be appropriate? The ones I've looked at are very flexible and depend on an AC plastic frame to hold them in place. The Magic-Pak is set up so that the filter sorta hangs so that wouldn't work too well either. It seems we need a filter that holds itself flat without a cardboard frame.
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