I have a Xena disk lock/alarm; ‘ got it cheap on eBay a while ago. Make sure you get the right size though.
The lock appears to be very strong (they’re heavy) and the alarm works good. The button cell batteries last for ages (they also supply a spare set and a storage pouch with the lock ).
‘ Only problem is that the alarm goes off for no reason sometimes (common to all of them I believe). If that becomes a problem for you, you can actually remove the alarm mechanism from the lock and use the lock on its own.
The alarm has actually saved me a couple of times, from riding off with the lock still on the wheel (which could really have ruined my day). To be extra safe though, you should get one of those orange coiled string things to remind you that the disk lock is on the wheel.
If you have a bike with front drum brakes (vintage bikes or the current Honda Nighthawk 250), you could use a bicycle U-Lock.
Better security might be a chain or thick cable so that you can attach the bike to something immovable as even disk locked bikes can be lifted onto a trailer. A real pain to carry around though.
Bottom line, if they really want it, they’ll get it, but if you’ve taken at least some precautions, they might move on to the next guy’s bike instead.
Always set your fork lock whatever you do, even at the gas station if you have to go into the kiosk. It only takes a minute to run the bike up a ramp and into a waiting van.