Fix the sonos beam to your woodboard
Try to align the front with the corner, so you can use this wihout further cutting.
Protect your sonos
Tape a thick paper to the part you want to copy.
Clamp a stop peace
If you like a straight line at the back, just clamp a piece as stop block to the desired position.
Save your router bit
You can cut the outline roughly with your jig saw to ease the wear of your router bit.
Route the outline and
Move your router with a copy-bit all around your sonos - remove the sonos - and sand it smooth - maybe you want to oil your master piece?
I just used a white metal bracket to screw the Beam in place - Enjoy!