What I am thinking at the moment is using your idea for using the existing holes to mount it but using the bottom ones and centering it below the plate. I make stuff like this all the time but lack of spare time keeps me with a back log of more items to design. I have ordered the parts so when they get here I'll sit down and start measuring and modeling. I'll probably keep it pretty simple. If there is room I may try to use the top mounting holes and try to fit it above the plate some but I'll have to see if that would look okay.Would love to see what you come up with!
I might be your first (and possibly only) customer for one.
I just made a simple bracket out of some sheet aluminum I had lying around the basement.