Mounting Triplanar Tonearm to SME 20/2 Turntable

I could also have a wooden disk cut to the same size circle as the SME arm board and attach the TP arm to it. I don't want to detract from the aesthetics of the system, but I suppose I can paint it black so it blends OK.  

This will result in reduced performance. Here's why:
One thing that is really important in any turntable is the fact that vibration occurs in the plinth. If the vibration is different in the arm mount as opposed to the platter surface, it will be interpreted by the pickup as a coloration. So the armboard should be of the same material and as tightly coupled to the plinth as possible. Anything else will introduce a coloration.


If I were you I would get a blank arm board from SME and simply drill the three mounting holes for the screws.

This will result in reduced performance

.
"Will"? WILL??? 

Or, "I think it will". Or how about, "According to something I read somewhere it will." Or better yet, "I'm guessing with no real clue that it will."

Apprently someone didn't read the post above. Either that or doubts what was heard and written. Because if it really is so important for the arm to be coupled then why does it sound so much better when its not? And if its really so important for materials to match then why does it sound so much better when they don't?

Another thing I notice, conspicuous by its absence, is any mention of having actually made comparisons and learned from experience. Not one word along the lines of this is based on experience.  Instead all we have is unsupported unsubstantiated opinion.

Right? Right???

My suggestion is make a new aluminum arm board for your arm.

https://www.etsy.com/listing/194959127/aluminum-round-disc-circle-6061-250-14?gpla=1&gao=1&&...

https://www.gcaudio.com/images/uploads/777/sme-20-diagram_square.jpg

@atmasphere Thanks for your advice, Ralph.  I appreciate it because I know you own a Tri-Planar and it was your recommendation on these forums that I began researching them.
For the record, the plinth of the SME deck is not the same material as the arm board.  The arm board looks like machined aluminum.  The plinth is something else, I'm not sure the material.  
I'm considering making the existing arm board work, but haven't decided yet how.  Tri Mai says he can help me figure this out if I want to go this route.
I've also contacted SME about ordering a blank arm board from them.  I'm assuming the price for this will be more than I want to pay.  They charge $9 for one of their own hex wrenches (standard metric sizes), as an example of how over priced their accessories are.  One can only assume the arm board will be hundreds of dollars.  That said, if going this route is the best way I will spend the money.  
@imhififan Thanks for the link to the etsy listing.  That would likely work just fine and be very affordable.  
@millercarbon Thanks for the suggestions.  Much appreciated.  I like the idea to experiment with different materials.  

Three choices. #1 buy a new tonearm board.
#2 laminate 1/2" MDF to the current tonearm board to cover the hole. Spray paint it black and make sure you can lower the new arm enough to get it down to platter level. 
#3 Make a new tonearm board. Layer plastic and aluminum layers to get the right thickness using slow set epoxy. Use the old board as a temple to router the board to exactly the same shape. 
If you don't have the stuff to make your own just buy a new one.