I did something similar with my Yammy BB415 - actually, I got the Bass Doc to do it - he routed out the back cavity (oo -err Missus!!) a bit, routed a new cavity for the battery box and bored the holes for the switches.
He also painted the interior of the cavity with not one but two coats of conductive paint & made sure it was earthed, included in the price.
This Yammy is now a Tone Monster - thanks Howard, smashing job (Grommet, I'm desperate to say!!) mate.
G.