I've always been happy with the Raxxess metal sliding shelves. They slide smooth, they have a lock that clicks in when fully extended, and the shelf itself sits real low in the space so all but the tallest pedals still fit within two rack spaces. I just don't know that you will find one for a shallow (less than 14") rack. Good luck, and let us know what you find.
EDIT: Upon further investigation (trying to find the part number) I have found that the shelf I like appears to no longer be made, and the replacements all seem to put the shelf at the top of the rack space (perhaps they can be flipped upside-down?) or use a laminated wood shelf.
Now I guess I'm really interested in what you find!