Updated: May 15, 2021
I've tried working with several different kinds of treat-dispensing systems and I've observed many students -- work with them in classes, in a working session (therapy dog), and while working in and enjoying the outdoors. BTW, I own no stock in any of these companies and have not agreed to any endorsements; merely my opinion on product function.
I've yet to find an airtight treat pouch that keeps fresh treats soft. If you want to leave treats in the pouch until the next walkie-walkies or training session, place them into a silicone reusable bag or some other kind of airtight container because if you don't, they will be rock-hard next time you go to use them.
My favorite so far (see below image), but I bet I know what you're thinking, "that looks like a fanny pack!" You could make that argument but please observe that it's a flat-fitting belt and you can't beat the function. Hands-free, sleek and adjustable to your body so you're not constantly bumping into it, you can hide it under your
shirt or jacket (so your dog isn't radar-locking on the bag and trying to get at it), zips closed, waterproof, fits most phones, pockets for poo bags, and removable carabiner(s) to attach other items like a leash or whatever. Our trainers often wear these in classes to keep their hands free, and they come in a variety of colors. (https://www.doogusa.com/products/doog-walkie-belt-navy-yellow-improved-design)
When it comes to dog training treat bags, I prefer a sleeker bag as opposed to the old school cup-style pouch -- I think it best to avoid the pouch that holds everything but the kitchen sink otherwise you'll be fumbling, risk stuff falling out (phone), and your attachments may bounce and clang around as you move. In the case of the cross-body bag, when it's heavy it can swing around into your dog's face when you bend over. To each their own though; you might have your reasons for carrying more. The following are recommended as slightly better in design than others; note that they have both a cross-body and belt clip option, a method to seal off treats from spilling out yet treats are easily accessible and the bag is compartmentalized so that you can keep your stuff clean & dry:
This bag (above) is an example basic style that is available from many different manufacturers. Just look for the essentials we talked about earlier and you should be fine. Wear it slightly behind your side so that when you move your arm back, your elbow doesn't knock it around. Wear it on the side your dog will be walking (again, slightly behind your hip and your leash held in your opposite-side hand) because if your dog is going to target the bag, then it should be in the place that you want her to be. If you placed the bag on your front of hip then she may target and forge (be slightly ahead of you) as you move with her, and if you put the bag on your opposite side from your dog it will likely cause her to cross in front of you or behind you to target the bag.
Or, you could use a sandwich bag, or even better for Mother Earth, a reusable silicone bag stuffed into one of your pockets (though you will lose some ability to quickly retrieve treats without fuss--timing is critical in dog training). I recommend not pulling that bag out of your pocket while training, waving it around in your dog's face as you fish for goodies--if you do you may find your dog's "obedience" become dependent on the presence of the bag.
My (personal) preference is that the dog I am training not be able to readily track a bag; rather, a reward suddenly comes from me, encouraging the dog to sort out where to be or what to do to earn the reward rather than focusing on a bag.