I looked for a good (affordable) one for a few years and stumbled upon this about 3 years ago. Red Wolf Airsoft sells a great quality copy - I bought one - does what it's supposed to do, looks good, and fits tight. Don't let the fact that it's airsoft turn you off - it's very well made.
So here's the deal. You have to follow their Red Wolf's web-page every day for a while as it seems they get them in batches. They go from out of stock for months, then in stock for a few weeks, and then out of stock. BUT they do get them - just takes a little persistence.
You can also sign up for a stock alert, but I didn't want to have to sift through my "spam" e-mail address every day that I use almost exclusively for online shopping.
Maruzen P99 Under Mount Rail
Product Code: MRZ-4992487990348
Price: $10.99 each