Let's start off with the MOTOR. I'm using windshield wiper motor, but it is a stupid idea. Well not stupid but a beginners mistake. It's heavy. So heavy in fact that it pulls the whole ExoArm from your arm. I used it because I had ideas of using this ExoArm to lift 10kg and more. What I did not account for was that this ExoArm needs back support if one would wish to lift this kind of loads. But then it wouldn't cost $100 but more like $200.  

So if you are going to make this arm, I suggest you try the motor that lifts and lowers the car glass window. It's lighter, it has lower torque, but for our applications it's good enough. 

Something like this: https://www.ebay.com/itm/Power-Window-Motor-Front-Left-ACI-MAXAIR-83098-fits-98-11-Lincoln-Town-Car-/253215691421?epid=75818760&hash=item3af4d4d29d:g:DrgAAOSwno1ZpYq8&vxp=mtr

But I strongly suggest that you visit a local car junkyard and buy one there. Who would spend $50 on a new motor if you can get used one for $20. 

Second, how to fix your arm to the ExoArm. If you look at my design, you will see that the only supports that I used are on the bottom and on the side (wood plates and foam). I encourage you that you place a support on the upper part as well. It will make your arm fit tighter to the ExoArm and therefore make it more intuitive.