I use a cheap Jim Dunlop one - they are available for curved and flat necks, and work equally well on 6 and 12 strings.

It's a simple piece of bent metal with notches in, a nylon strap and a plastic lever. There's a piece of rubber between the metal and the strings. I've been using them for over thirty years on my Yamaha FG260... I have a Shubb, but find the Jim Dunlop far more convenient, and tuning is rarely a problem.

I always carry at least two of them in my pocket (someone always needs a borrow!!).

Hah! Here's the website!! They show both the simple one and the adjustable one - I prefer the simple one, as the adjustable one has a habit of readjusting itself when you're not looking!