If you don't intend to provide Hifi like quality (which I assume from the speakers shown) I would definitely go for the Buzzer style PCB mount speaker. Using that will be waaaay easier than mounting the other speaker on the case.
If you have to use the external one I personally would stamp a few holes into the chassis to create an opening for the soundwaves to travel to your ear and just press the speaker against the chassis, glueing its perimeter with an elastic, strong silicone glue.