In PnP games, it's common for DMs and players alike to allow the casting focus, if it's a simple or small thing (i.e. you're not a wizard using their staff as their casting focus), too presume it can be fastened to the wrist easily and safely - so that players don't have to think about it overly much. Casters can do whatever they like with their hands, and when they cast they can just be presumed to flick the casting focus up into their hand - the same hand that they use for somatic components anyway, which is allowed in the rules.
Remember also, if you're discussing having a free hand for casting needs, all that actually means is that you need to be able to have the hand free when you cast, not that it be free at all times, just as two handed weapons only need you to hold them two-handed when you're actually attacking, not that both your hands be glued to them at all times... so if you're using a two-handed sword, for example, you actually *don't* need to put the sword away to cast, because you can take one hand off it while you're casting.
Working with casting foci is actually a lot less cumbersome that it first seems (and it does seem cumbersome at first glance), and that's a flaw with how it's described and set up, I think. Mapping that flexibility into a game, without *making* it cumbersome, however, is trickier, and I don't blame games that just quietly do away with the need for a focus. To do it smoothly, it should really just be as simple as noting that various things count as or can be used as casting foci for various types of casters, that you need to have one (component pouches in this case would just be another type of focus) that's appropriate to your casting class in order to cast spells with material components, and beyond that, it should just need to be on your person *somewhere*, but ideally NOT taking up an actual equip slot - I'd prefer as a separate equip slot that is specifically for a casting focus, so it doesn't interfere with the rest of your gear, though even that can run into complications in niche cases (one character with multiple casting classes with incompatible focus needs; though even in that case, the player in question can just use a component pouch, since that's a universal).
