I have a nice shiny one like yours and two very old and very abused ones that I got before I got the shiny one.
I haven't seen a mounting plate either. But there have been several discussions about these large trackballs and mounting other trackballs in general. But there are a few old threads that may help. Search for "duckpin," "candlepin," "trackball mounting," and "giant balls" amoung other key phrases. I spent 10 minutes looking for this
thread for you. There is a picture discussing the differences between a "High Ball" mount, a "High Lip" mount and a normal amount that should apply to your 4.5" ball in the same general way it does to a 3 inch ball; at least I think so.
Oh, and check out
this thread here, I think the bevelled edge brings a nice touch.
There is another thing to add on my list. I wanted to get an obnoxious sized trackball to play Golden Tee with, and now that they are added I guess I should get on that.