I myself will use rubber tape, because it is thin 1/16 and you can build up to the thicknesses required, also the type I am using fuses itself to the frame and motor mount so that it will not move under engine torque, which I believe is the cause of mount bolt breakage. It can be removed OK after removing the engine.
You can get it at any hardware store. It is used to seal pipes that are leaking and under pressure.
I would think that leather or rubber not fused to the frame and motor mount would move, no mater how tight you get it, but that is only how I see it, not the way it might work.
I don't say your way won't work, I just thought you might consider my idea before you try it. The only way to know is to try it.
Good luck in your efforts, let me know how it works.