marcdi: I would recommend you purchase the WA/Solarc 10W ballast before doing any work.
FYI: Bore from the head at 1.378" ID and 1.625" deep. The deeper the better (more reflector options), get as close to the retaining clamp as you can while leaving space for the wires to sit btw the switch and ballast.
If you're making a MiniHID then bore all the way down to the retaining clamp. There is no need for it.
Happy Modding!