This - - is what people without darkrooms can use. It lets you pull out enough film and cut it (carefully, so you don't cut the bag) and tape it into the tin. If you want to get fancier, you might be able to figure out how big a piece you want and make a guide for cutting to that size. Then you can have clear photo corners (like for scrapbooks) already stuck into the tin. That might be easier than having a roll of tape in the changing bag.
Good luck and HAVE FUN! As for developing, you'll need an option for that because you can't usually take an altoid tin to a lab (though some might be fine with it). I'd advise using B&W and learning how to develop it yourself.
And the changing bag can be used to get your film from your Holga and Diana into a developing tank to do those yourself, too.