Only trouble with using those T nuts is that they are designed to be used with the points facing the load point, not away from it like the way that you are using them. You might get more holding power if they are glued into the knob, but I would have put the T nut into the hole in the cap with the points facing toward you. Then screwed a bolt of the correct size into the T nut and coated it's threads with a thin layer of grease to keep the epoxy from sticking to it. Then fill the hole around the bolt, covering the T nut with epoxy and filling the cap hole. When the epoxy dries, if you have completely coated all of the affected area of the bolt threads with grease, the bolt should unscrew, leaving a very strong threaded hole for you to attach the knob to whatever needs it.
If you epoxied the T nut in place in your knob it might hold, but it won't be nearly as strong as if you installed it the other way.
This is a great way to make knobs from old bottle caps, if you can get the epoxy and the T nuts easily, but my way should prove to be considerably stronger.
Central North Carolina