I have also used the following to carry my light and a spare:
http://www.paracordsecrets.com/products/36/product.html
The AA version is great for carrying the Draco! I use it to protect mine from other stuff in my pockets by inserting the light lens first. Note that a little heat and a "squeeze" is required to mod this product to ensure it holds the light firmly. The AAA version carries the spare battery effectively, but it can be difficult to remove the battery... especially in the dark!
The kydex products work well, but my favorite way to carry is as shown in my previous post, as I prefer to carry my Draco and spare batt in my pants watch pocket. The kydex battery holsters add too much bulk to do so comfortably. Metal "spy capsules" also work as a spares carrier. Just ensure that you add some non-conductive material in at least one end!
modamag: Thanks for the response. My original thought was actually to suggest an end cap for each end of the AAA extension tube. That way I would have the tube, extra o-rings, and a spare battery on me. Doesn't hurt to dream, right!
Side note/question: When using the extension tube my light won't fire up on a 1.5v AAA, only on a 10440. This is normal, right?