Excellent How thick is the aluminum?When you run your motor on land is it noisier then when in the water under load??
The aluminium section will be whatever thickness the hardware shop sells. Usually sold in 1 metre lengths of extruded section.
As pictured, the mount is just using the treaded section at one end, the rubber section appears to be just hanging there, doing nothing. Motors are mostly intended to be rigidly mounted to a bracket using the two screw holes provided, then the bracket is used to spread the fixing load. Usually a screw in each corner. If the rubber decouplers are used, they go under the bracket feet.
As suggested earlier, a layer of rubber sheet between the underside of the bracket and whatever it is fived to will do the job.
Model boat hulls do tend to act like musical string instrument bodies - any sound inside them gets amplified.