Tips
The larger the straw (in circumference), the louder the sound will be.
Experiment with different size bottles, holes, straws, different types of balloons, and the tensions of the balloons until you get the sound you want.
Use a heated screwdriver to make poking holes in the bottle easier.
Making the hole on the side larger will make it less of a strain on your cheeks to blow into it.
The straw should be longer than the bottle itself.
If it's not working, make sure the straw is against the balloon on top.