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.