System doesn't have a place for a filter

I have a Goodman GMS81155CNA furnace configured for upflow. The return ductwork in in the crawlspace. Each room has a small baseboard return grill.

When we bought the house, I went hunting for the filter. It was placed in the bottom of the blower compartment The filter, 20x25, fits the opening perfectly. However, there is nothing to hold it in place. When the fan is running, it lifts the filter up several inches up against the blower assembly. Air flows around the filter, defeating its purpose.

I've searched and can't find a solution. Is there some type of pre-fab filter rack that can be installed in the bottom of the furnace? Do I need to fabricate one myself? Or do I need to remove the entire unit and install an external filter rack on the floor under the furnace?