[MENTION=413168]GSJon[/MENTION]

The Felt items I have are based on the U-M 1.1 Aero:Road seatpost and are approximately 20 mm at it's widest point and 41 mm long. Now the number plate holder is a bit more flexible than the GoPro or Garmin mounts so it might be possible that it will work, but since I have never seen a 3.1 Aero Post I could not guarantee it. I had spoken to Felt about a year ago asking if they would send me broken or cut off seatposts so I could measure them for things like these and they declined saying their designs were proprietary. The even went so far as to say that their included reflector mount was good enough for their customers. I have not been back to ask them for anything since. Use this information however you see fit.

If you were willing to part with your seat post for a few days and were willing to ship it to me, I could make up a number plate mount specific to the 3.1 seat post, I would send you back your seat post a few days later. If that is something you wish to consider just PM me here (Shapeways messaging system sucks and I usually try to immediately redirect all inquiries I get there to other messaging systems I actually use).