I went up and read the last few posts. Let me update it a bit. I found that rubber bumpers are not the best answer to keep the faces separated. The better solution is using small pieces of Velcro, either the hook side or loop side, doesn't matter. I have used both the sticky back Velcro and the glued back Velcro, set down with ca. The Velcro separates the donut facings about 1/16" which is fine. When installing the rubber band I squeeze the facings and Velcro separator firmly. Works great....
I recently made a couple small versions of the hyde mount for a twin engine scratch build that was just vibrating to holy heck. The engines were small (.36 asp 2 strokes) but the high speed vibration felt like it burned your hands just holding on to the airplane. After a couple throttle servo failures and bolts from the landing gear dropping out in flight, I looked into soft mounts.
I did use a piece of rubber between the front and rear halves because that is what I saw in some other diy threads. Is there a specific reason that you changed your practice?
My mount used the rear of the round hayes motor mount for the front disk and a piece of 3/16 birch ply with a 1/4" balsa glued to it for the rear half. The mounts reduced airframe vibration by at least 90%. I just was curious why you are using Velcro. Should I be concerned and looking for something to go wrong if I used a disk of rubber instead?