I was going to suggest a Shimagnolo cassette, a Shimano cassette respaced for Campy, but the only one that can really do that is the Ultegra 11-28 because it has loose cogs. All of the others have the 5 or 6 large cogs fixed on a spider and only the small cogs are loose. You can't add the spacers to the fixed cogs. On top of that you need an 11sp Shimano splined hub. A 10sp hub is just a hair too short to hold the extra spacers. I've been using one like that on my trainer, which was 11sp, and since it is a trainer it doesn't have to be perfect either. It shifts well enough. On my wheel I have the usual 13-29 Veloce cassette, the widest one I could find. On those rare days I hit a big hill I always want lower, but as it is most days I don't ever go to the small chainring.