any lock that will fit in the slot will, er, fit. just make sure its a nice long and wide one, short cycling locks arent much use for motorbikes. shackle locks are categorised by the width and length of the D shaped locked area if you know what i mean, the slots in the luggage rack will fit a variety of sizes but the underseat bit is a bit tighter.
i got a sold secure silver rated ABUS lock with a nice long shackle for £35 in a cycling website, and it fits under the seat and the luggage rack fine. the only thing the honda lock comes with is a rubber strap that goes in the rack and holds the lock down, stopping it rattle about. easy to make yourself, i will take pics of mine if you like.