As an alternative to a spacer, which moves the lamp horizontally forward, I eventually standardised on the B+M 471LH, a fibre-filled rubbery mount which raises the lamp vertically, high enough for its light not to be blocked by the fattest tire you can imagine. It is immensely stable, with none of the micro-tremors found in metal mounts. And long-lasting too. One of mine, bought c2000, is still serving faithfully. Only downside is that it is Bauhaus functional, not bling for bikes that make a tasteful statement -- I doubt the surface will even take and hold paint.