A great solution to the problem and well done. The aluminum of these storefront frames is quite soft which lets the rivnut 'dig in' making their use excellent here. Some of the cheap tools sold for using these fasteners wear out or break fairly quickly so if you do a lot of these consider spending for one of the better tools. If you rarely do these a cheaper tool should be sufficient. I prefer to use a small dab of grease or anti-seize on fasteners going into rivnuts because of that rusts or corrodes, on trying to remove the fastener later the rivnut can spin, then you can't even drill it out because that will spin too. For less strenuous fastenings into these frames always use fine-threaded fasteners and don't over-torque them; with soft aluminum less is more.