Downflow furnace filter question

Good afternoon.
I am trying to clean and reinstall the filter on an older-model downflow furnace. I opened the access panel for the intake plenum, and found a pair of (very dirty) washable type filters that had fallen inside the furnace. So much for filtering efficiency.
The plenum opening is roughly 17" x 17". There are a pair of filter elements, each 17" x 10". I see no obvious way to install them so that they are held securely in place. There is a crossbar that clips in place across the plenum opening. The bar has a "W" profile and is about 4" wide. I assume it has something to do with securing the filter elements.
Any suggestions on how to secure these filter elements in place?
Thanks -Mark
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