I sometimes fill the bottoms of smallish boats with hot glue to get them to float well.

Pros: Glue spreads easily, fits crevices, and is pretty cheap and easy to use.

Cons: Bit messy, not particularly heavy (though does make the prints feel more solid), bit of a pain to sort out how much glue to use and at what intervals while printing.

Can’t say whether glue would be a good fit for CNC machine parts though.