The complex metal frames really aren't necessary, but do look nice.
The factory filter is just the folded paper material w/ a cardboard strip on either end of the folds (anchoring the folds). A proper sized foam filter wouldn't really need any frame edges.
The size would be 8-1/4 x 7-5/8, w/ the narrow part being F/R and the long sides being L/R, when in place.
Some have just got a 20x30 house filter from the store and cut it down to fit, making 6 filters from a sheet.
I'll probably just cut to fit and stick it in there, no frame needed. But . . . like I said before . . . the frames look nice. Plus, they would be more accurately fit for those w/ allergy issues (less air slipping past filter).