I have no idea what the sensitivity of the information stored on this warrants this level of concern by you, but as long as you don’t want to resell or reuse the HD, you can do just about anything you want to destroy the data permanently. Magnets, microwaves, impact maintenance, chemicals, fire.... the world is your oyster.
Personally, I like to use a hammer and nailset to poke a hole through the HD casing and break the platter without sending metal and plastic shards all over the room.
Won't putting a hard drive in a microwave start a fire?