How much space have you got between your rack and the seatstay? I saw an Instructable at the weekend which described how to attach a couple of lengths of PVC conduit to the inside of the rack struts using jubilee clips. Looks like a neat solution, but I don't have enough space.
I've just bought myself an Abus Ultra d lock, and I'll be sorting it in my rack top bag, as I can't use the supplied bracket without removing both water bottle cages.