Think a little less, so you can resaw 4/4 lumber to make the panels and save some money. They're not structural, they just block dust intrusion. Remember to float them, and if you're smart, prefinish, so you won't see edges revealed when the humidity drops. You can buy these, as many do
http://www.rockler.com/space-balls-rais ... or-spacers , or get some spline material to do the same job.
http://www.homedepot.com/p/Phifer-0-210 ... &gclsrc=ds keep a fat one and a skinnier one on hand. Fat along the grain, thin across.