Hi Kushan,
Eyro comes with a mount for the fork. I removed it and bolted Eyro directly to my front fork's 3-pack mount. (I might have used a spacer, I don't remember offhand.)
For the Soma rack, you likely would need a "brake arm" or similar piece to attach the Eyro in its proper position (after removing the fork mount).
Eyro has been great for my evening commute in the winter. It holds enough charge, is simple enough to clip/unclip for charging, and, importantly, has a nice lens with cutoff. I sometimes would use a MUP, and the lens cutoff is great for keeping the light out of users' eyes.
Also, the mount makes it look integrated, and not clip on. This might be a deterrent for thieves in contrast to handle-bar mount styles.
Good luck!
shoji