I attached the FastenR-3 lug for my two BR straps to a Kirk QRC-1 1" clamp
, with a little Loctite Blue 242 on the threads. That way, I can easily move the attachment point between body and lens plates, or remove it for tripod/monopod mounting.
For an 'upside down' application like this, there's no strength advantage to the wider clamp, so I prefer the Kirk 1" to the RRS B2-FAB-F, which is 1.5" wide. Here's what the Kirk clamp looks like:
Thanks for the blog link, Kent - RRS was late to the game on this one, but that's a useful reference. When I was first looking for a clamp solution, I called RRS for a recommendation, and at that time, they didn't have the -F version of the clamp (-F means flat). The original B2-FAB that they recommended to me has metal nubs on the bottom that interfere with attaching the BR lug. Their advice when I asked about the nubs was to, "Just mash the rubber washer down on the nubs, or file the nubs off." I bought the Kirk clamp, and some months later, RRS comes along with a flat-bottom version of their clamp. That's been my only poor experience with RRS, otherwise their stuff is great. But for this application, the Kirk is a better choice for me (which is why I bought a second one for my second BR strap, though the -F was available by then). FWIW, the second BR strap is a left-handed sport strap, and that Kirk 1" clamp on it is holding up a 600/4L IS II with a 1D X on it - needless to say, I trust it!