Just do the boxes; standard size (whatever you decided that is) and then offer custom sizes for a premium. Make them quality and deliver what you promise. The market will always make room for someone who can do that. People will always make something cheaper because there's always a market for junk.
As far as pre drilled and kits.....no matter what kit you make, they'll want something different and if you try to be all things to all people you'll end up doing nothing well. If anyone is on the ball enough to make a mod, they can drill their own holes. I mean, it's a freakin' hole.
That's my take and I wish you much success. Under promise and over deliver....works every time.
I can not wait for these to be ready I have plenty if stuff to drill holes but know nothing about making the wood nice I'm in on this big time . hopefully room for two 18650 and a reg.