That's a good idea if you don't want to damage the kayak, but I meant
that the diameter of most scupper holes only accomodates an easier to
cut cable.

A friend who ran a livery a few years ago used a very thick and long
Kryptonite cable that threaded through the scupper holes of a bunch of SOTs.
This seemed to work OK --never had one stolen. The cable is plastic covered
so no additional insulation was needed. There are Kryptonite cables and
locks available in varying lengths for bicycles. Perhaps such a store would
be a good source of something you can use.

Joe P.