I've been using 3/8" anti-fatigue mat from Lowes:
http://www.lowes.com/pd_261481...__?productId=3457892
Here's a photo of what it looks like on my layout before ballasting:
I cut strips off the mat with a box knife, then bevel the sides on a bandsaw with a fence in place to keep the strip aligned.
I drill holes on either side of a tie and use a zip-tie to hold the mat and track in place. If I need to move the track I simply cut the zip-ties.
Lowes used to sell it on a roll, not sure if they still do. At the time I bought mine, I figured it was cheaper than cork roadbed.