Personally, I don't like plexiglas: it tends to melt when you drill it or carve it with a Dremel.
When I lived in the city, I used to go to TAP Plastics, and grab chunks of plastic sheeting from their scrap bin. ABS is very nice, and expanded PVC is pretty easy to work, too.
Now that I'm a country boy, and the supply I accumulated is dwindling, I sometimes have to get a little creative. Like using the container from my blood pressure meds as a mounting base.
For temporary set-ups, I also use Lego to build bases that can hold even complex constructs securely.
Threaded spacers are ridiculously expensive, so I made my own: if you buy 10 or more of these from Digi-key, they're only 6.5 cents apiece, and the hole is almost exactly the right size for tapping 4-40 (It's close enough that you don't need to re-drill them). The savings on a couple dozen will pay for the cost of the 4-40 tap. Digi-key also sells bags of 100 4-40 screws and nuts that are much cheaper than even the big chain hardware stores. Just tack them on to your order when you find you need some fancy chip or other part you can't get from your local Radio Schlock.
The rubber feet won't really do much to help with vibration: they're usually hard enough that, for the tiny load an Arduino puts on them, essentially all the vibration will be transmitted right on through. What they can be good for is anti-slip protection: you'll often find that connecting cables weigh much more than your Arduino set-up, and can easily drag the whole thing off the workbench if jostled. I have one of my Lego baseplates mounted to a piece of 1x6 lumber: nothing short of my tripping on and dragging a cable moves that sucker ;D
Ran