I took a couple pictures this weekend of how the setup would work with the accessory bars and the Frontrunner brackets. The brackets come with M8 bolts but the whereas the Frontrunner rack just uses the slot to hold the hex head bolt, the Rhinorack uses M6 slot nuts to bolt on accessories. I ordered some M8 slot nuts but they are too fat to fit in the channels. M8 slot nut on the left, M6 on the right.
I could just use M6 bolts and the slot nuts but because the RTT is a large object that I don't want to become a projectile in an accident type scenario, I'm going to drill out the M6 slot nuts to use as a spacer in the accessory bar which will keep the hex head of the bolt in the channel and prevent it from turning. Here's the drilled out slot nut:
An then of course the bolt, slot nut and bracket attached. The Frontrunner bolts are only an inch so I will be swapping those out for 1 1/4" so the nylock nut has enough to do it's job:
Hope this helps clarify a few things. Is it overkill? Perhaps, but I'm in the I'd rather be safe then sorry camp, so the little bit of extra work is worth it to me. YMMV!