Do not, I say again, do not use serpa's. They are trash. Perfect example of why not to carry one (this guy is a idiot anyway so it takes away from my point but, you get the picture.)
http://www dot youtube.com/watch?v=zYvAxLX6OzE&feature=channel_video_title
This was posted up on a bunch of mil forums within the last few days. Try safariland if bladetech doesn't work for you. Both make great holsters. Also another place to look is raven concealment.