In the pictures you see a Ronstan RF67 360 degrees swivelcleat (max 8 mm line) and this works beatifully. I use 6 mm line for my traveller and that is perfect for my boat and I-track traveller rail (F16). I can't imagine any boat no matter how big the mainsail would go over 8 mm line. (25.2 mm = inch so do the conversion yourself)

Also note that the foot piece for the cleat is nothing more than a standard 3 mm alu extruded L section (not anodised, that is not really needed) that you can buy at your local hardware store. One side of it was bend and hammered into the beam shape by the builder. It used (from memory) 3 monel rivets to keep it into place. I'm quite satisfied with the end result.

I hope this helps.