I have used cutting boards (HDPE) as a baffle by just attaching it to the door sheet metal using short wafer-head screws, covering the area near the driver. The 3/8” thick panels will conform to slightly curved surfaces without forming.

There was a study that I had read about where someone had tested a multitude of different materials for use as a baffle, and HDPE stood out as the winner. I can see how it would have a deadening effect.