Raspberry Pi sells little add-on boards that fit on top of the tiny computer. This kind of board is called a hat. Pardon me, it’s HAT for “hardware on top.” C’mon, that’s freakin’ adorable. Anyway, the newest one is an add-on M.2 slot just big enough for the smallest standard drive (a 2230, 30mm long). That’s the same one that goes in a Steam Deck.
Raspberry Pi already had an M.2 HAT board for slightly larger 42mm drives, but that one was more of a full-on expansion. This one is small enough that it can screw down and plug in beneath the height of the various connections of the Pi itself. That means that, in the absence of any other major additions to the Pi 5, you can fit the M.2 HAT inside a Pi case–specifically the official case sold by the Raspberry Pi company itself (the cutout is for the fan module), but it should also work with plenty of others.
The Compact variant of the HAT+ board fits into the same FPC connection on the Pi 5, as noted by Liliputing, making it an easy integration into most builds. And while 30mm M.2 drives are typically a lot more expensive than other options, it should be a fast way to add lots of extra storage. The new HAT is on sale now from various suppliers.