Wet\dry sandpaper on a piece of tile, glass or even a flat counter top will work since your not going to need to flatten the stone to much. Wetting the back of the sandpaper will hold it in place, but I've used double sided tape on the two long edges to hold it. The Naniwa stones are pretty easy to lap. The DMT can be expensive for occasional use, but if you start getting into bevel setting and that the diamond plates are invaluable. My Norton lapping stone sits on a shelf collecting dust since it takes more effort keeping it flat than it does to other stones.