I have been using the Panther Marine lock. They work well and are fairly cheap (about $25). This is handy, since the two I have had both had issues with the lock cylinder freezing up. They might have done better if I had tried to open the lock once or twice during the season and lubricated it, instead of waiting until November.  While the cylinder was easily drilled and the lock removed, the shackle is proprietary so you need to buy a whole new unit.

Does anyone have a suggestion for a motor lock for my transom mounted outboard motor? Thanks.