Changchung,
Well, wouldn't the size of the head be purely based on the size of the reflectors you want installed?
Easy enough to figure the internal diameter needed if you can specify the diameter of the reflector, or optic, that you want to use.
Say you were using the IMS 20mm, which is actually 20.5mm, then you would have this:
Measure the height of the reflectors, LED, heatsink, lens, O-ring, etc. and you have your minimum depth.
Thickness of material is determined partly by thread depth, plus personal preference.