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