Are there any basic hints and tips to designing custom gearboxes?

I was planning a 2 stage reduction of 3:1 - 3:1 with the Technobots MOD1 gears, 12 tooth and 36 tooth.

Will I need bearings, how big should the walls be, do you bolt the gearbox together, how can you save weight etc

I am guessing you aught to have this kind of thing CNC'd so if there is any companies or people who will accept a solidworks file let me know

Thanks in advance