ready made speaker mount adapters for 2012 Prius?

I made adapters out of MDF to install Alpine 6x9's in the front door of my '12 PIP and 10 months later I opened it up and found that the material is absorbing water. I will be ordering plastic adapters before fall.

 

MDF can be sealed with a primer like Gripper or Kilz to help resist water absorption

 

I am going to replace them with PVC adapters.

 

I've read some reviews of these, and would probably consider some 1/8" aluminum plate, or I might just make up some 1/4" thick fiberglass sheets (since I'm laying up resin on a regular basis)