What better than a piece of aluminium? Maybe with a very thin gasket underneath to ensure a firm and secure fixing. Cut out a circle big enough for the mounting points, then drill through the middle enough for the arm to go through. Nothing particularly troublesome there. At the moment I've not needed to use this spacer, so it's purely theoretical....
To me it has one very large advantage for audio equipment over PVA; which is that it expands to fill any gaps. So if the two pieces of wood being stuck together have any imperfections that could allow air through the glue will seal it. See the picture below for how the glue expands enough to ooze out of the sides...
This was a real result, it was used for well over a year on my original turntable to effectively reduce noise. Making the component was simplicity itself, just use silicone sealant to glue the three materials togehter, then I glued a black edge strip to make it look nicer. Here are some photos. The finihsed sub-base One of the feet glued...