It's really quite simple to open up a Hubbard squash or any other big tough squashes.
Put the squash in a paper bag, lift it up high, and let it drop onto a hard concrete floor. I went down to the basement and dropped it from the ceiling. Once the squash has been cracked open, it generally can be cut into further smaller pieces without much ado.
The various suggestions to hit the thing with an axe, a mallet, or other sharp or blunt instrument, or even to use a knife to start the process are quite inefficient compared to the wonderful force of gravity that can be at your command in this process.