Ball of Death: or How I Learned to Stop Worrying and Love Apple Pro Speakers

Step 5 How to Eyeball like a pro

Stick the shaft of the motor through the wire hole on the back of the core. If your motor is too shallow, shove a bottlecap under it to prop it up.
See how gravity kind of "wants" to center it? If your motor has 6 holes, line up your target holes along the plastic "beams" and the other 3 hole along the curves of the outsides of the "beams". Now, using  flame and a big nail, burn holes through the plastic directly above the threaded holes in the motor. Try not to go all the way through. The last thing you want is molten plastic dribbling into the motor. Use your reamer-like thingy to file out the holes.
Tough part:
A lot of this project is subjective. I just happen to be a packrat when it comes to tools, fasteners, and gadgets. I just happened to have the perfect size bolts for the right size motor for the speakers. It was a personal trifecta in my case. Try to do your best with what you have.