Bought a $9.99 cutting board from Walmart. All I did was cut the cutting board to the width I wanted. Centered and traced the base-plate on the cut piece. Drilled the mounting screw holes and counter sunk them. Put my up-spiral bit in the router then mounted the new base (cutting board) to the router, made a small cut into the cutting board then measure from the outside edge of the hole to half of what size the cut-out is I'm needing. Keep in mind to keep the line for the measurements centered. I measured from the straight edge to the center of the bit and copied to the opposite end of the board. Then used a laser line to draw the line and mark my spots for 8-12" woofers. (Don't forget to drill out the center of the board for the router bits)
This is near exact for me as I was within a 1/32" of what cut-out I needed on each woofer. All you do is make your center point for the woofer, drill into the center point (not all the way through the wood), then detach the drill from the bit while its still in the wood and slide the new circle jig onto the drill bit. Be sure to use the same size drill bit that was used for the holes drilled on the router plate.
Speed and SQ