

Bigger hammer and a concrete surface. Three good whacks to the thin sheet metal casing (opposite the drive motor/PCB) should shatter the platters inside.
You can also buy a sharp punch that looks like this and punch thru the sheetmetal side to really get those platters broke.
Realistically if they’re already failed, nobody is going through the effort to send these disks through any kind of speciality recovery for a random john q public anyway.
Adapter or caddy is fine. Can get them on most shopping sites for cheap.
IIRC from my old office PC slinging days, a lot of those cases with 5-1/4 bays usually had slots for mounting screws that would allow you to mount a 3.5 drive flush to one sideusijg 2 screws. Then you get a 1-3/4" 6-32 screw stand off, thread it into the drive, and use that to mount it to the other side of the 5-1/4 bay.
Did that a lot to really old reused cases where there were a ton of 5-1/4 bays but only one 3.5 bay.