How can I fill two or more adjacent spaces on a QO panel?
That's a common complaint. Every maker's filler plates are a) flimsy, b) expensive, and c) hard to find.
Fortunately, Square D also sells a much more robust filler plate which solves all these problems, called the QO120.

They're sold everywhere, latch onto bus stabs, and aesthetically look - well, they actually are breakers!
I'm not joking.  &quot;Actual breakers&quot; really are my go-to for filling empty panel spaces, because it solves the flimsy problem, and they're just more available.  My local lumber supply stocks these, they do not stock blank covers.
As for cost, since I'll probably be fitting a circuit in the future anyway, 120V/20A breakers are a pretty safe choice because if you're adding a circuit you can always choose to use #12 wire.  And they can be handle-tied for 240V/20A circuits which do not involve neutral.  (though a handle-tie is another procurement bug-hunt).