adding big cog to end of campy cassette?
 

I have 11 speed campy cassettes in 11-25 and 12-25. I would really like a 28 or 29 big cog.
While looking for a deal I noticed on ebay they are selling a Miche Campy 29t Final Position Cog for 10 Speed


Curious if pulling a middle cog from my 11 speed cassettes and adding a spacer and the 10 speed big cog would work. I don't think the 10 sp cogs are much wider than the 11... anyone know?

Campagnolo 11-speed cogs are 0.1 mm thinner than 10-speed cogs. For just one cog, especially one at the end of the cassette, I don't think the minor difference in thickness would be a problem.

One other possible consideration: The shift ramps on each cog are lined up in particular places relative to the next cog. You may notice a difference in shift performance if you remove a cog from the middle of the cassette.

...and don't mix up the spacers. They're not all the same thickness across the cassette.