When I improved the sound in my former 190D and added some quality speaker to the rear shelf, I changed the complete rear shelf.
I made a shelf myself that was a bit higher as the OEM one to gain some depth that made it possible to mount quality speakers. The height between the steel chassis and the OEM shelf is too low for any good speaker and the home-made shelf made it even possible to mount the speakers in a stealth way (I covered the shelf with one carpet-like speaker clothing).
I preferred the higher shelf over a cut chassis...I have mo experience with Honda parts
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greetingz,