I finally went backed to the source and reviewed Dave Everett's styrene plans to see what he did. And lo and behold, right on the original plans were schematics for where to mount the dome motor! Doh!
I printed out the template, got things mapped out and after a few passes with the old Dremel tool I got things ready. I reinforced the spot on the frame where it mounts with another layer of Sintra. I have seen the area on other builds reinforced with a thin piece of metal, but I need to scrounge something up to use for that.
I mounted the motor with two screws for the moment since I have to remove the lazy susan to get at the third hole spot, but for now it seems very secure with just two.