There are about 40 machines to do, too, so foam core is attractive in that it is easy to cut, but I've also been considering little sticky backed rubber shoes.
I would go with rubber shoes. McMaster-Carr
has a great selection of adhesive rubber feet. They come in a sheet of 20 or so (I think, it depends on the size of the feet). We ordered some of these at my work for a project that needed to stand and they work great. Just search for adhesive backed bumpers (or rubber feet) if the link doesn't work.
They probably would probably be best for airflow, too, since you're not filling the whole area.
Might even reduce vibration, which would be another plus.
(Oh wait, SSD's don't vibrate.
Edit: Post 666!
If you're getting 404'd when trying to use the links in stickies, try this tutoria
l to find that juicy info.