The belt clips I purchased at the local Tandy's Leather store. You can buy them individually packaged, although they are a little pricier than I think they should be. Here's a side view of the clip which may clear up its design a bit. This was on a leather sheath I made a while back:
The clip has one end with a ramp to allow easy-on. The other end has a squared ridge to secure things.
Basically, with a two inch hole spacing on the sheath, I used a design template to draw a 2" wide oval on kydex. Cut it out and shape it as desired. Next, I marked the holes to match the 2" spacing and drilled them. After that, I positioned the clip, laying it on top of the kydex oval and marked the width, centering it between the two holes. Next, using a drill bit, I drilled a series of holes between the two marks which represent the width of the clip. These holes were on a line and close to one another or overlaping, so I could connect all the holes with a file to make a slot the width of the spring clip.
This slot should be sized just smaller than the square ridge. Place the kydex between the clip, lining up the square ridge with the slot, heat the clip, and press it through the slot.
Secure the oval to the sheath, and the clip is secure.
Be glad to answer any more questions you may have.
--nathan