Made a little sketch of what the spacers need to look like between the ribs to ensure just the right amount of space for the arms. These need to be .505 thick.

...and here they are! Chris was nice enough to let me scrounge through his aluminum stock where I found a piece of 1/2" thick 2024 plate. It was about .510 - .515, so I used his mill to machine it down to the correct thickness.