Make your own with cheap snow pants, cut out the length you want around the knee and move up the elastic inner ankle retainer to above the knee (depending your leg size, it may be possible to just cut off a length and turn them upside down). Wear them over the top, take off if too warm, slide back on if too cold. I have the retainer at the bottom instead of top, but my legs are more or less sticking out forward on the recumbent so it's also a wind seal for me, if upright then you may want elastic at the top or on both ends.