I was able to attach the cable release in portrait mode without any adapter. See attached first photo.
However I called RRS to discuss best way to attach my Black Rapid strap to the L-Bracket and they suggested a screw knob clamp- see second photo- which was about $50 or $60. It is very solid and does a good job but not inexpensive. They said it was not advisable to attach the strap directly to the bracket plate.