Yeah that's good idea, thanks for the suggestion.
I even went out and bought a puzzle book while building this, so it's definitely not something most people have around.
Just FWIW, if you need a supply of sudoku puzzles, the wonderful Simon G. Tatham's (PuTTY author, for Windows users) Puzzles page has a lot of algorithmically-generated, guaranteed-solvable puzzles, including sudoku (called "Solo" on his page).
I'm partial to "Patterns" myself, though it definitely needs a mouse or trackpad (preferably the former).