A solid baffle is absolutely important. Many cars don't have smooth, flat surfaces for the woofer to mount to, so a spacer/baffle/ring is needed to ensure that the woofer is mounted solidly to the door without gaps.

 

This is important.

 

concrete strong speaker base is a must for good sound system - solid rattle free authoritative midbass,...

 

HDPE speaker rings are a waterproof alternative to MDF.

Something like this cutting board can be used to make them.

 

HDPE is great material for speaker rings. If you want to build let say doorpanels or sails, I wouldnt use HDPE since it will not bond with fiberglass or epoxy,..in that case plywood, ALU, steel.... would be better pick

 

Main reason for A speaker ring is to provide a flat surface where to mount it. If you want to transfer the energy from the speaker to the baffle then that baffle needs to be heavy and rigid, how to do that is by adding mass and connect it to multiple points on a wider surface. Deadening material is not going to stop vibrations as easy as slab of concrete since is not rigid enough and heavy, is going to lower the resonance frequency instead