I used to wear a fanny pack often until I realized that it takes too darn long to present from the pack. You can carry IWB a G21, I know, I do it everyday.
I have carried my 21 in the small of my back IWB. I couldn't sit without pain though.....very uncomfortable.
My uncle mikes side kick pack is easy to get to the gun. Heck with good situational awareness its faster than most carry methods. I just pop open the gun compartment and slip my hand onto the gun without anyone the wiser.
Heck if not for my fanny pack I would probably carry a little gut popper with some of my Florida clothing.