Jonathan,
have you been copying my design ?
Only kidding. Mine is very similar, basically the same shape but without the rounded off corners seeing as I made it on the milling machine then lathed out the inside.
I have just cut two more alu plates today at work and I am going to make two more spindle mounts and see if I can get two identical mounts that are a better fit for the spindle than the last one I made.
My original plan was to make two as my first job for the cnc machine but I am still having problems trying to get USBCNC to work properly and also my workshop pc died earlier tonight. At least the weather was nice today!

Ian