I've attached a stainless steel mending plate (available at home improvement stores) to the front threaded braze-on of the SOMA porteur rack. The headlight is hung from the other end. The headlight is well below the rack, so it's not in the way of cargo or tie-downs, and is protected by the rack from being hit by anything; it's offset from the wheel, and so out of the major water spray from the fender, and it's inside the rack's vertical supports, so it doesn't interfere with hanging panniers. The length of the mending plate isn't too important; you just rotate it back on the braze-on until the headlight is between the wheel and the rack verticals.

The IQ-X is, IIRC, the only B&M headlight that can be rotated in it's supplied bracket, so that the bracket can be inverted while the headlight is right-side up (which is important, because it's not sealed on the bottom).

I'd attach a photo, but I am not yet allowed to do so. :-/