You could probably mount them directly to the metal, but there is no need. The plastic insn't brittle so I don't think it will break. I have Polk DX6s and I have them mounted that way and I have for about 5 months and they're still rock solid. Also, there's a better chance of the speakers getting wet with water in the door if you don't use the rings because they will mount much further into the door.

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Eric Konarske & Lara, my X
'01 Solar Yellow XE 3 Pk. 4x4