> An eggcorn is a soundalike though, isn't it?

Not necessarily, you might be thinking of malapropisms but yes probably a closer word would be the general term: protologism.

Another commenter added some useful info on the evocative alteration of metaphors [2]

1: https://en.wikipedia.org/wiki/Malapropism

2: https://news.ycombinator.com/item?id=33757097