I have a gently used defnder neck brace for sale. I bought it in 2010 but used it once or twice until mid 2014 when I started racing.
http://www.defnderneckbrace.com
The brace was recently recertified in Feb of 2016 with replacement of the pads, straps and tethers, and new sfi sticker dated Feb 16. Includes helmet hardware (threads into sa2015), 2 super awesome defnder stickers, padded case, and 2 wrenches.
I've never used another brace but I found this one comfortable and secure. I don't remember if it was a "one size fits all". I am 5'9" and have weighed between 220 and 190 (depending on what I'm training for) with a 17" neck. I'm selling because after 10 years, I burnt out on thrashing to try to beat corvettes in st2 with a camaro. (Another looooong story) . I put the racecar in storage in march and started racing in triathlons. Whenever I come back around to racing the sfi dates will likely be expired and I'm certain someone could use this in the meantime.
$400 shipped from 78233
Email me at spatrick30 at hotmail dot com for pics. (Snafu with photo bucket account)