The common failure of these mounts is that the rubber delaminates from the mounting plate and the currently available mounts do not last very long before they fail.
As a result I have been attempting to re-bond some of the original failed mounts without any success but yesterday I used some gorilla epoxy and it seems to do the job. I tried it on two mounts. One I had managed to completely de-bond the rubber from the mounting plate but the second one was in the main still attached so I cleaned it up the best I could between the rubber and the mounting plate. I then clamped them up tight after putting gorilla epoxy on the surfaces that I wished to bond. This evening I have attempted to break the bonds but have failed. Previously I have used a number of different glues, including gorilla glue, but I have been able to break the bonds. It think it is well worth giving it a try so do not throw out your old mounts. If you do try this please provide feed back.
Another approach I am looking at is to use a Land Rover Defender mount type ANR 1808.
It would require a 15mm thick mounting plate with a M10 tread tapped in the centre to make up the distance. See my wooden mock up in the photo. What do you think of the concept?