I did a skirt for a metal stand once, it was two pieces held on with rare earth magnets from lee valley:
http://www.leevalley.com/en/wood/pag...=1,42363,42348
The 1/2" ones will be enough, I seem to recall using the 3/4 and they were a bit too strong, was a little harder than ideal to pull the skirts off.
As for plywood base, personally wouldn't bother with it unless the tank sits flat (no plastic trim). If it's a standard tank with trim just use some foam or even some thick rubber if you're worried about what it looks like. Most tanks I've seen setup including the one above didn't use anything, straight on the metal, never heard of any issues before.
If you do use plywood, make sure it's good plywood and properly sealed, I have seen problems occur from such material swelling up from moisture.
Oh and whenever possible with stands or skits I make them cover up to just above the sand level of the tank. This hides the hard to clean algae and makes cleaning the glass with a magnet easier as you don't have go right to the sand.