405 backbox sizes ?

[ayden]

Hi guys
I have searched the forums but to no avail. I would like to stick a "beefier"[:D] backbox to my G-reg 405 gr, i have found a couple i like but i dont know the size of the backbox ive got on it now. Im assuming that the one ive got on there now is the right one for the car but could someone please tell me the biggest box i could fit in to replace the original without causing any problems ? thanks Ayden