I actually just bought their 30L bag for a weekend road trip last week. I struggled a little bit at first for attachment points, initially thinking like you to remove the fender stuff so I could get the straps under the metal supports (under the pillion). Then I decided to simply use the GT back support bars (right below the back support) for the rear straps. I wrapped them nice and tight around those supports and just rolled up and zip-tied the excess strap (for now until I decide to cut it nice and short).
For the forward straps, I dropped them down under the main seat and wrapped them around the silver sub frame directly below your butt (and right above the rear foot pegs). I had to extend the straps to nearly their full length, but they come out the side of the main seat and are far enough back that my butt doesn't interfere with them. They are completely hidden by the side panel body work at that location and also fold back under the seat when not in use to be completely invisible.
The bag was extremely secure for the long ride and turned out to be a very nice lower back support. I'm very happy with the end result, the only minor down-side being that the rear straps are exposed when not in use (unless you take them off every time) and have the possibility to show some wear marks on that brushed aluminum back rest supports. Personally, they're are small enough and blend in with the black body work well enough to not bother me.