Interesting that you should ask this as the same question came up a few times this weekend at NAHBS. The BikeCAD booth was right across the aisle from the Columbus booth. Unfortunately, there is currently no way to enter a specific tube cross section such as that into BikeCAD. Of course, if you were joining that to a round head tube, you could obtain a perfect miter using a hole cut saw. If you're joining to a tapered head tube, or don't have access to a mill and are therefore determined to use a miter template, I'd think you wouldn't go too far wrong approximating that tube as an elliptical cross section. However, I have not actually tried it, so I can't be sure that such an approximation would be up to your standards.