Picked one up almost brand new at local consignment shop for $40. Doesn't slip, no freakin' billet straps at all, and best of all has the little bit extra memory foam/cushion under the saddle that we need.
Ecogold's are great, just be careful washing them. sometimes they fall apart in the washer.
You could also try one of those neoprene or jelly-like wither pads. Its just a rectangle, may or may not have holes in it. they help a lot with slippage. i like the jelly best because it is sticky. sometimes it loses that, but just put some water on it and the sticky is back