use the template you got for the speakers and place them on the shelf with them closer to the back window ..
use a black marker to draw the outline of the speaker
get a jigg saw and cut out the shape of the speaker
you should miss the matal bar with no problems ..
its not hard to do , just think about it
The speakers on mine are 6 x 9.
The job was not done especially well but they could not be fitted without cutting through that stiffening bar. So I put that strip of steam bent wood under the back edge. No more rattles.