P-clamp with a long bolt (probably 40-60mm or something, and lots of old v-brake spacers to get it in the correct position, maybe some serrated washers if you're feeling fancy. Just the actual bolt going through the dynamo light mount hole, assuming it's a standard one which is 90° to the direction of light. It can actually look pretty tidy if all lined up nicely.
P-clamp with a long bolt (probably 40-60mm or something, and lots of old v-brake spacers to get it in the correct position, maybe some serrated washers if you're feeling fancy. Just the actual bolt going through the dynamo light mount hole, assuming it's a standard one which is 90° to the direction of light. It can actually look pretty tidy if all lined up nicely.