I doubt you can :-) They will grow into the shape they want and attempting to alter that would likely damage the gourd and cause it to rot. The good news is that most will develop that hourglass shape naturally.

Awesome! the first gourd for right now anyway doesn't have that shape (pollinated about 5 days ago) and the next one that'll probably bloom tomorrow has closer to the shape I want, even though it has a neck. However I will see what happens. Worse case I keep growing this plant until I get what shape I want lol. Its my favorite so I have no issues with multiple seasons :D

Let them hang naturally without being constrained by anything else. Not all will develop the perfect shape but quite a few will. A friend of mine grows them on a trellis. Even the ones that are a bit crooked have some character to it.

Just as an aside. Japanese grow watermelons in a box to control its shape and you guessed it they come out as big cubes. It is a big thing for them. Eg. https://www.buzzfeednews.com/article/mbvd/japan-begins-selling-cube-watermelons-to-customers-overseas