I applaud your choice. You are only going to find two types of retention holsters for such a great OC piece: the one you found and styles with a simple loop that goes over the hammer spur. I think you are better served with the strap, which should be fine for OC retention purposes. The comfort of a holster is one of my more important criteria, so I hope you find one that you can use comfortably, especially with such long sidearms.
But you are required to have a permit to OC a BP in TN? And were you required to be 21 to purchase a BP, or was that just because you were waiting until you could legally get the permit? In most places, I think you can mail order them at 18.
Welcome to the forum. I think the biggest disadvantage to your choice is the stability of the powder charge if it remains loaded for long periods of time without being fired, but I am NOT an expert on BP firearms.
Personally, I've collected a couple Ruger Old Army revolvers to use if laws were to limit me to such a weapon. Other than their weight and size, there is no denying the effectiveness for the task.
EDIT: On second thought, though they are not typically labeled for the BP guns, a good holster designed for a modern single-action cartridge revolver should serve you. For example, I have a Safariland vertical shoulder holster for a 7.5" Ruger Vaquero (the original large frame ones). It is comfortable for both OC and CC (the latter in the winter only!) Many holsters for a large six-gun of the same barrel length as your BP gun may work quite well.