I was concerned about the plate thickness, but Walmis said it was plenty and that turns out to be true.
I didn't realize this until I took the first pic above that the profile that the Rhino plate attaches to is sagging (it's not the plate). I just tried to tighten everything up but it's still doing it. I'm contemplating putting something under it that sits on the floor to support it (not that it matters, other than visually and to my OCD's insistence on orthogonality).
But I think you need a short piece of profile that mounts to the front of the bottom-most profile in the picture (you marked the latter red) and then the Rhino plate would attach to that. IOW, what I did but centered on the frame rather than sidestick-style. Maybe that's what you are intending but if not, I would not mount the plate directly to the top of the profile. It might be fine, but I do think the plate needs some support for at least part of its front-to-back length, as I have done. Either that, or put something under it for support that goes to the floor.