I think its best to stick to the same material as the plinth. Mine are all black acrylic. Metals might ring, so I would stay away from those.
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I'm not sure if you're looking to make a decision on a turntable or just make a new armboard.
I disagree with Cmk. There are more 'tables built with/from metal than any other material. If you took a poll you would probably find just as many, and perhaps more, people recommending to stay away from acrylic. The answer is what ever sounds best to you.
The armboard is certainly an important piece, but I believe the platter material will make more of a difference in the majority of sound that you hear coming from a turntable.
Good luck in your search,
I have been told acrylic/plexi etc for unipivot arms and brass for gimballed or other arms. I would suggest with you table to try a few. The entire analogue chain is about synergy and you will need to experiment to get different listening biases fullfilled. Cartridges will make the most difference as far as I am concerned if you design your system to be neutral.