Downflow furnace filter question

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