I have some thoughts about this project, Riona.
First, I'm WAY impressed. It looks like a great idea and your design ideas sound really good. Waterproof--an absolute necessity. Permanently secured by the tail piece mounting bolts--BIG improvement over anything that could be jimmied off. Lockable--another absolute, and using the third OEM lock is a big plus.
I like the look of the "PPS" concept; it looks a lot like the Hyabusa, a cool design IMO, and I also like the idea of making it large enough for a helmet, or really for much of anything, since I think the original Smuggler is way too small to be of much use for anything. Just an opinion.
As to getting your leg over, why not just test the size and shape with almost anything--a cardboard mock-up, a sack of rags, a small dog (if you can train him to stay)--and see how it works. When I do a camp trip, my back seat holds a big strapped-down waterproof duffel that's a lot bigger than your model. My normal mounting procedure is to stand next to the bike and swing one leg over. Can't do that with the duffel, so I have to either get my leg over the seat from the side (usually have to grab my pant leg and lift for that last little boost, but that would be a problem for someone much shorter. I'm 6'1") or put my right foot on the right peg and swing the left leg over from that position. That gives me the extra height I need to get over.
Finally, I really like the name you picked. But. I don't think Corbin can say much about you doing something similar to something they do. After all, how many companies make tank bags or tail bags? I just hope if you started to sell a few, you wouldn't run into any problems that might be made worse by picking a name that's reminiscent of the name they use? Any patent attorneys on the forum?
But anyway, Riona, I think it's really great. And since I don't carry passengers either, I wish I had one too.