Hi Mark,
The Hi Hat you got from MSE is the wrong size. The standard for Moy / Mitchell bases varies a little bit, but it should be a minimum of 7 inches to a maximum of 7.25 inch. 6.5 inch is too small. Since you have bought both of these pieces, I can recommend 2 things. The first is to move the key on the garfield mount closer to the center. That way even if it shifts it will not rotate. The other is to make a new washer and lock combo for the garfield mount, which has a step milled into it that sits into the inner hole of the hi hat, so that when you tie it down, it cannot slide from side to side.
Hope that helps
Sanjay Sami