Simple as...Get a small clean plastic bucket with a sealing lid eg glue bucket or gibstop compound.
Throw a layer of PLAIN SEASALT over the botom,(never iodised or the crap with free-flowing agents in it!) add a layer of pre-cut bonito , another layer of salt, more bonito and so on.Put a final layer of salt over the top of the lot and sit the whole mess somewhere out of the heat.
You add water very gently (without washing the salt off everything) as you layer up your baits but only in small amounts as you fill your tub.... and make sure the water is HEAVILY salted.
All that salt stops any bacteria dead and your hunks will turn into bullets that any self-respecting fish will gag at as soon as he tastes it.
But you did say you wanted to harden up your bait.
Whether the fish accept it is up to them.
Good luck.
PS: You can re-use the juice as part of the next batch too.