The motor adapter pieces finished printing, and I have attached one of the motors.

Here is a close up of the actual adapter piece:

And this was my quick and dirty way of mounting the motor to the rest of the assembly:

I hooked up the motor to my Arduino for a quick function test, and it moves! There is very little torque, but it is just fine for a plotter. I doubt I will be attaching a Dremel or anything like that anytime soon, however :) Next up, I have to design a better way to mount the motor.