Heathkit Hero Robot Wrist Connection - Tighter Fit

This is a remix of the wrist connection part shared by the Swindon Makerspace for the Heathkit Hero Robot. I found their part to fit rather loosely on the arm tube of my Hero, so I modified the design a bit.
Changes include:
* I reduced the diameter of the central hole to make a tighter fit on the arm of the robot.
* I added small holes above each nut slot. These make it easier to reposition the nuts when installing screws, and make it possible to eject the nuts altogether if you need to.