Grace g-940 arm mount????

If you have an existing hole that would work at all I would use that. As you point out the arm does not know how big the hole is, it is held in place by the pressure from the top and bottom of the shaft, not the side. Use good quality washers on top and bottom of the board , I wouldn't worry about the bushing, unless I am looking at it wrong I don't see how it would be essential. I would try the current Linn felt mat, I just bought one myself. $25 + ship. from the Linn shop on ebay.It is directional , one side up sounds better than the other. I use to sell several of the heavy rubber mats in the days when I was a Technics and Micro dealer; just sold the last one a few days ago. They look impressive but current research suggests that the thinner and deader the better. A prominent French audio expert has come out with one made of calfskin. There is at least one made of carbon fiber, both these last ones cost far more. If you want a traditional mat the Michell that I just sold was the best I used. Then there are the copper ones like the TTWeights one; I haven't tried it but their other products are good. Try the Linn first, you may well save yourself a lot of time and effort. I liked the Star Sound threaded brass cones so well I became a dealer for them. I replace the feet on the VPIs I have on demo with them, makes a noticeable improvement. Others are also available, I obviously haven't tried them all.