First what kind of laser are you going to be making the host for? 445, red, bluray, maybe a green module. Each of these require different heat sinking depending on how powerful you want to make it.
Batteries are a big factor in how to make a custom host as well. Batts come in all shapes and sizes. So figure out which size works for how big or small you want your host to be but will still supply your laser with enough power and hold enough for a decent run time.
I would figure out what kind of laser you want and get the batteries that you will need and use the batteries for measurements.
Since this will be your first experience with a lathe I would make the host to fit a standard aixiz module for simplicity. And don't forget about space for a driver cavity.