I think that washable stuff gets flimsy as it gets old. Mine did the same thing
I just happened to be able to grab some pictures of mine
This is one of those cut to size filters from the hardware store
like this one, except I'm not sure of which filtration level I bought
Web 25 in. x 30 in. x 1 in. Eco Plus Adjustable FPR 4 Air Filter-WP2530FPR - The Home Depot
I'm pretty sure the little clip rings came with the filter, meant to clip the filter element to the black grid
As you can see I clipped them through the AC's diffuser grill
I mentioned this to you before, but for others' benefit, I would suggest not getting a very high filtration level. I found that mine restricts airflow a bit more than I'd like.....and yeah, I can see it's a bit dirty now
Anyway, my suggestion is to get some hardware cloth or something similar, along with some new filter media that's just a bit stiffer then the old stuff and zip tie it like mine is....