All these trackballs are essentially Betson Universal clones. Their design allows for flush mounting with a few modifications and the right bolts.
You may have a better way but here is how I did it. Thanks to Franco B for the inspiration via his JLW/JLF top mounting procedure (which worked fantastic btw!).
Step 1:
Ensure trackball is centered. Best way to measure this is by placing the trackball upside down and resting the flat portion on your CP to see if it provides you with enough distance from buttons and comfortable hand rolling.
Step 2:
These trackballs are 3" trackballs but require a 2 7/8" hole saw bit.
Step 3:
Once the hole is cut I place the trackball in from underneath and line up the base and then trace it for the routing.
Step 4:
You want to use a straight bit and route down to within 1/4" of the top of the CP. Remember when routing by hand go clockwise around the perimeter or counterclockwise when within the frame (coin door etc). If you are not sure of doing it by hand along your pencil lines use straight edges clamped to your board to keep the router in bounds. My first pass is usually within the pencil mark so I can work into a tight fit. A snug fit will balance the load on your bolts.
Step 5:
I then mount the trackball and screw through the trackballs existing bolt holes so the holes are bang on.
Step 6:
For flush mounting I used these hex bolts (3 1/2") from Home Depot or other hardware stores as they are perfect length...have a nice flat head and a nice diameter to spread out their surface area.