The bigger the gun the harder to conceal no matter how you do it. Carrying a particularly large or heavy gun in a fanny pack or similar setup can also be a stress on your back. If you carry a smaller mid-sized to compact gun (maximum 7" length x 5" height, and thinner is better but not critical) you might want to take a look at some of the carry "pouches" that look a lot like a camera case or soft PDA holster. These usually go onto your belt rather than having their own belt and allow you to carry in plain sight in what is essentially a closed holster right on your hip. One of the companies that makes EMT (Emergency Medical Technician) hip packs for basic things an EMT might need and they modified one and made it into a concealment holster. So it is the same pouch with two totally different applications. I do not work for them but I like their product. They are called Hawkepak. Another option is to severely modify a real camera bag but of course the most critical element is the gun be securely in the holster pouch and the holster pouch be securely on you. I would not use those cheesey little belt loops that come on most similar pouches not marketed as for carrying a firearm.
Back to fanny packs - the bigger the fanny pack the more awkward and uncomfortable it is to carry, plus you would obviously have a large fanny pack for a larger gun making things even worse. Also, though where I live it is quite common for all sorts of folks to wear fanny packs, carrying and non-carrying alike, it still can look a little odd when someone is wearing a particularly extra large one. Doesn't mean they are carrying but it does tend to catch my eye faster when I see a fanny pack that is larger than my grandma's purse strapped around somebody's waist. But that is the great thing about an armed society isn't it? I bet the crooks don't like playing the guessing game either - maybe they just have a lot of crap to carry. I rarely carry in a fanny pack anymore but I strongly preferred the zippered ones rather than velcro to get to the gun (quiter and actually for me faster). Get the smallest you can get the gun into without stretching it in such a way that it looks like there is something trying too break out. How far out it sticks is a huge factor in being noticed so try and use a thinner gun and do not put other stuff in the other pockets. That is bad form anyway. I used to do that and went to get my wallet one day and accidentally pulled the wrong zipper and just got lucky that no one saw and freaked out at the store as the butt of my gun got exposed for a second.
Personally I use the belt pouch (EMT style) at 3:00 (maybe 2:47) with a PT111 and I forget it is there but if I need to get in it I can do that extremely fast. I can actually squeeze a Sig 229 into it but much prefer the smaller, lighter PT111 and there is tons of room to spare then so the pouch, which is medium sized at 5.'"x7.25", looks half empty with a smaller gun in it. Just like fanny packs, avoid the bigger belt pouches as well. Carrying anything bigger than the 5x7 standard, even slightly bigger than that, is IMO too big to comfortably, or even really inconspicuously, carry in any 'plain sight' manner. And a 5x7 is even pushing the absolute maximum at that, at least for me it is.
