I know this goes back a few months but if I can recapture your interest, i'd be interested in better understanding how you've fitted this and how well it's gone. I've bought the same thing and i've fitted it permanently using the velcro strap under the seat (removed it to fit). Mine's a little forward of yours, not on the rack at all - a little too far really, causing me to be unable to sit back. The tail pack instructions are very clear about using both sets of 4 bungees. Maybe you have some mount points that I don't? I don't seem to have any below the seat. The only option that was available to me was to hook them from the bag, around the bottom (in front of indicators) and then hook to the luggage rack on the opposite side to get the tension - I did this for both the underbag ones and the ones to keep it down when loaded, e.g. 8 bungees crossed under. When tight they're no risk where they are, well clear of the wheel, but I've found one unhooked after a journey already, luckily not dangling, but it scared the hell out of me. I can't see it ending well if one of these hooked around the wheel at speed.
I'm inclined to think it would probably be fine with just the velcro which is quite substantial, but i'm an inexperienced rider so could do with some insight from others. I went this way after reading that boxes affect handling, but i'm starting to wish i'd just gone that route anyway.
I might be judging too quickly. Certainly yesterday everything stayed hooked up, and when doing the hooking it's very hard to see how any of them could become unhooked, but I suppose if my body pushes the bag around it might happen.
Any insight appreciated.
Cheers,
Tom.