Securing HVAC air-return filters to the return plenum

Our heating system uses "standard" 25x16" filters placed inside the bottom heater compartment over the return plenum
However, there doesn't seem to be any mechanism for mechanically securing the filters to the return plenum opening; instead, the filter just "leans" freely against the opening without any type of seal or mechanical attachment.
What is the right way to secure the filter so it does not "flop" around and if necessary where can I get the required parts?
The heater is an American Standard Freedom 90 (Model AUX080C942C3)
Note, there does appear to be a lip around the side of the heater cabinet attached to the return plenum that may once have held some type of rod that went over the filter [this is the way one of our heaters (though another brand) seems to work]
Thanks
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Likely there is supposed to be a stiff wire or rod to retain the filter. The manufacturer or dealer might supply one or you could fabricate one pretty easily. Don Young

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