I take it that'd be the Keyser which is made for a rounded rather than flat keyboard? I believe they have several variants. It's incredibly frustrating having a capo that puts you out of tune, but I've found that the rounded version works well with my Angelica.

And the bent-nail-in-a-rubber-tube version works pretty well with the Portuguese mandola, and that is a really curved fretboard, almost like a crescent moon.