Suehiro Deluxe Stone Holder
Without a doubt this is the best waterstone holder on the market. It's large size provides a wider footprint, it contains tackier rubber, and more weight than standard sized stone holders. This, in turn, provides a rock steady (extremely sturdy) platform from which to sharpen from. You put this where you want it and it stays there.
This model has one less screw/nut to deal with (than standard two screw models) which is very nice, however, I've found that when switching stones (while working out of one holder) that it's completely unnecessary to even bother with the screw/nut because the rubber holds the stone in place from simply snugging the holder ends up under the stone - what a great time saver!
A bonus feature of the Suehiro Deluxe is that it will hold all stones - even those with bases up to 250mm in length. For those of you who have been shying away from buying stones that have bases (like the Naniwa stones) you won't need to shy away anymore, your stone buying options will now be wide open. *See the attached image for how this works
I've been using this stone holder for some time now and I won't ever go back to the lesser models. The Suehiro Deluxe is what you've been looking for.
Holds stones from 150mm - 250mm in length
Overall Size - 230 x 85 x 37mm