Not sure about the how of your mount, but a few years back, I rode with someone who carried the in a similar way. His holster was about 3-4 inches further back, diagonally pointed down at about a 20 degree tilt toward the front axle. The holster did not actually mount to the head tube, but rather the top tube and down tube. This kept it a little farther back, so it never contacted his fork if he had to tilt his wheel all the way to the side (track stand, etc.). If I recall, he got some type of normal kydex holster or full holster with a solid backing, and used some hose clamps like this:
He drilled a few holes into the holster and lined them up with the clamps on the tubes. A little rubber stripping kept them from rattling or damaging his frame.
On a side note, he actually had to use his firearm on the way back from a group ride. Some hick had a big Rottweiler that went after him. The dog's owner grabbed the dog's collar after it tried to bite my friend, and after looking at him for a second, let the dog go and yelled "get him". My friend dispatched the dog with a single shot. Fortunately, there were a few witnesses that corroborated his story. The police showed up, and the owner actually got arrested.