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Sheet, plate.. 2mm thick. You want to close up those holes for better bass response. But I can see that is too high end. Just put Dynamat over them.
I would consider adding aluminium sheet to those open holes in the door under the Dynamat to make it even better. Or is that too hi-fi? :)
This is just what I wanted to see. Did you add Dynamat inside the door also?
When you start damping and building the doors take plenty of pictures. I'd like to see what can be done.