hey Sc-, does your M402 model fit an assembled 402? i'm guessing not, if it won't fit a 401 batt.
likewise you probably can't "cheat" and squeeze an assembled 403 into the "401" model, if that model's the same size as our cheapy 500-mAh boxes. even a 403's about a centimeter too tall. you'll see the dilemma with the lid once you get into it: a taller lid will cover as tall a cig as you want, but the hinge will be too low to clear it while closing, so you're looking at either a raised hinge (tricky) or cut a flap in the front of the lid which can flip up during closing. the quickie solution would be a slide-on lid at the height of your choice with some stuffing material everywhere but over the assembled cig slot so things don't fall out of their slots (you'd need to do that to any modified hinged lid as well).
let us know what you come up with!
and yeah, Shawn's got things pretty well covered and the universe usually punishes you for straying.. i think he's got elves out there checking on us, specially round this time of year