Most simply create holes or cavities from the speakers to atmosphere with no consideration of the shape or how the cross section needs to change.
Passive amplifiers, like the ones being designed and made in schools, use the horn principle and parts of the Wikipedia entry might help students create something with a better sound.
"Roughly speaking, the slower the flare rate, the deeper and lower frequencies the horn will reproduce for a given length of horn. For example, a horn area growth rate of 30% per foot will allow reproduction down to about 30 Hz; 10 times area per foot provides midrange reproduction; 100 times area per foot is used in high frequency horns." http://en.wikipedia.org/wiki/Horn_loudspeaker
Here's a design I created for 3D printing using a Swept Blend through several cross sections. I sized the cross sections by 'eye'. No doubt it's possible to scale them using a relation but I think I'll wait and use embedded Mathcad in Creo 3.0. The Component Operation 'Cut Out' was used to make perfect sockets in the holder to glue the horns in. All I need is a 3D printer to make one...