Step 3: Support Frame and Plexiglass
Flip the speaker over face down on the plexiglass and trace around with marker. My speaker had mounting holes around the outer edge so I used those to mark out the center of the circle. Just mark holes then use a ruler to draw X through circle connecting the hole marks.
A sabre saw with a metal cutting blade cuts plexiglass easy enough the only problem is melt-back. As soon as you cut past a place it melts back together! So cut around your circle leaving a groove of melted plexi. Then fill this groove with metal cutting oil and go back around very slowly adding oil when necessary. (If anyone has found an easier way please tell me!)
Drill a 3/8" hole in exact center of plexiglass disc. I used a bolt to chuck the disc in a drill and then clamped the drill using it as mini lathe to grind/sand down the plexi edges smooth.