I just received from Amazon a "Get-A-Grip Foot Pedal Pad" that really keeps your pedals from slipping around, on both carpet and hard floors. It's made by Martelli Notions, (
www.martellinotions.com) The pad was made to keep a sewing machine pedal from slipping, which is very similar to the little Yamaha FC5 pedals. It works great even though it's only 9"x 9". It fits one Yamaha sustain pedal and one FC5 (the sustain pedal hangs over a little in the back, but it holds just fine).
I only just now noticed that the company makes the same thing in bigger sizes (listed on Amazon as "Martelli No Slip Pad for Sewing Machine") The other sizes are 11"x17" and 12"x21". I think these are exactly same thing I bought, only bigger.