i stay away from the innertube idea. using cheap rubber to fill up the space will only cause problems.
one, it'll deteriorate. two, it will cause the engine to vibrate more, and without a solid base to dissipate those vibrations, it can cause your hardware to snap.
there's a ton of threads on here about solid vs. rubber motor mounts, and the solid seems to win out everytime. i mount all mine as solid as possible with no problems.
the easiest thing to do is make a shim out of aluminum or thin steel. you can do this with a piece of pipe split down the middle (like an old seat post or handlebars.) or cut out pieces from a sheet of metal and bend them around the frame. i've done both, and it works great.