I think it's a great idea,
I plan to do similar when I get the time.
For a sub woofer to be effective,
it's purpose in life is to move air.
If there is a panel in it's way,
it sort of defeats the purpose.
Do BMW not know this?
Maybe their brainy chaps from R & D
should read this forum from time to time.
Colour code them to the panel,
and this should be a great "functional" mod.
Can you personally audibly discern the difference between cover on/off?
One thing,
with such large holes cut,
you may have to reinforce the rear of the panel,
small aluminium angle should do the trick.

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