So I want to make a nondeminsional bag, but I want it to be in line with my character concept. As a monk, do you think I could make my monks robe (body slot item) into a nondeminsional item? I like the thought of my monk pulling all sorts of items from his sleeves like a magician
Ideally, you could make this like a Handy Haversack, with the same cost and construction requirements. If you want it to be your sleeves, then you could make it an "arm" slot item, or make it a "body" slot item instead. The downside is that, currently, the Handy Haversack is a no-slot item so basically you get the benefit without losing a slot. If you pick "arm" or "body" slot, you lose that slot for no really good reason.
For that reason, just getting a Handy Haversack is probably a better idea.
But, if you like the concept of pulling stuff out of your robe/sleeves, then a slotted robe item that works like Handy Haversack would not be game breaking. As a side benefit, you could ALSO have a Handy Haversack, letting you carry twice as much.
Of course, all of this requires GM approval, as always with custom magic items.
DM Blake has the right idea: Handy Haver-vest sounds like a perfectly fine item. Same cost as one, same capacity as well, just a different delivery.
Side Note: I read this thread's title as "Nondenominational Bag Question", which did cause me some confusion, heh.