Like most things CBG, there aren't really any rules, so whatever works, but I made one myself and here's my take: I built a three stringer, low E A D. Originally I used a piezo, like my "regular" CBGs, but it really sounded awful, unusable. A friend was dismantling a cheap bass he had and I managed so score a bass pickup, MUCH better. The other main thing is the pressure on the neck - my bass neck bowed quite a lot after a few months, I would do what you can to address this - perhaps a truss rod of sorts, or maybe a short scale? Mine is fretless, so intonation isn't a problem but the action is pretty high now up the neck. Another consideration, perhaps: I like to at least have a bit of a sound, acoustically, but not really achievable on a bass, unless you are using a suitcase or something!