If you are getting into the dark art of hand sharpening, then it isn't just a whetstone you should be getting your hands on. Grab a holder and ensure that there is no awkward slippage frustrating your technique. The Naniwa Stone Holder has a shallow grip to ensure capable sharpening on your stones even once they are super thin.
"If you are trying to keep a regular smooth sharpening motion, it is essential that your whetstone stays where you want it and, despite the wet surfaces, doesn't slip. If you are going to buy a whetstone, stick one of these in your basket."
This stone holder accommodates stones of the following size: length - 136-218mm; width - max 76mm; thickness - 5mm upwards.