No mods required to the gun. The holster must be purchased as a Glock model, and you can trim the tip of the main frame to suit the length of the gun as well if you want. Just makes it a little more compact and doesn't dig into your leg.
Murray Gardner, IPSC Store, Brownells are all good places to buy.
Last I saw at the Triangle Match, the IPSC store was there, and it was a bargain for a CR Speed WS model. Don't know if they have the Glock model though. Murray Gardner is a good bet for one.