My favored magazine holder for a spare G19 magazine is a Glock product, a Gen4 G19; yes a whole second pistol, with that magazine inside its grip frame.
My next-most-favored magazine holders would be the ones on my duty belt, one is a double Safariland carrier, with flaps, to look pretty, and the other is also Safariland, open-topped, attached with the QLS mounting system. Grabbing my duty rig gives me over 60 rounds, in one package; 15+ in the pistol, if it is in the duty holster, and 17 in each magazine. Donning the belt, and then grabbing one of my other weapons, will result in quite a formidable load-out. The duty belt is Safariland buckle-less, so I can easily fasten it at a smaller setting, to fit me when I am not wearing as many layers of clothing and armor.
I keep a Glock OEM 33-round magazine handy, in the bedroom, too. It is discreetly placed near the bed, in no type of holder. It is just sitting there. OK, so I am a slob, regarding home decor. I have concealed-carry mag carriers, too, but they are not part of the home-defense equation, unless I am fully dressed.
I have reinforced the front entry door frame. Two German Shepherds are inside the house, and two large, mean, loud, mongrels are in the back yard. (Their sire was a Great Dane, and their momma was a feral droopy-eared monster that rhymes with itch.) If they are barking with an angry tone, I want to get close to my Benelli M2, first. If there is time, and it sounds serious, I don an armor system, which is Level IIIA all-around, plus a Level III plate up front. Then, the duty belt is added. I might even add a Level IIIA helmet.
Yes, it took time and money to work up to all of this. Had I chosen an occupation other than first-responder LEO-ing, I might not have all of this gear. But, as highly-respected instructors Darryl Bolke and Wayne Dobbs teach, when someone invades your home, you are your own first responder. Obviously, some of my post will not apply to everyone's environment. This being a discussion forum, read by a variety of folks interested in the subject, not a direct Q&A, I decided to go into detail; like a cafeteria, everyone/anyone reading this can pick the menu items that seem to be a good idea.
I am no expert, but have worked as a first responder, for a big-city PD, for 33 years, so might have picked up some little knowledge along the way.