I have a North face shuttle daypack that is exactly the nominal dimensions and conveniently boxy. I’m 90% sure it’s fake though, so going with a ruler to a store or market that sells backpacks is what I’d recommend.

There is a company making a hard sided suitcase with removable wheels which can fit. I’d strongly recommend against that because the handle will take up so much of the already limited space. And there’s nowhere to put the wheels when they’re removed where it can still fit. Note that Spirit sizers are unofficially 18149 rather than the officially advertised 18148. Frontier’s bag sizers are truly 18148.