I use a bicycle security chain and a brass padlock. Hook the padlock through one of the holes in the outboard mount and one end of the chain, wrap the chain around the guard rail and back onto the padlock. Bought the chain in Germany where they are plentiful and cheap, bought the padlock in my local swindelry. Total cost about 15 euros.
Obviously wont stop a real thief, but then I doubt anything will. Have used this for six years or so and still OK.
There was an outboard lock review in one of the mags some time ago.
Each device was on the lines of a metal shroud covering the clamp screws and secured with an integral lock. There was, specifically, one for a Honda too.
None of those locks withstood a determined amateur with either a drill or hacksaw for more than a few minutes and some for only seconds.
The conclusion, I thought, was that such devices largely met the requirements of insurance small print, in having securely locked one's outboard when unattended, when submitting a claim!
Cost of devices was high, in some cases nearly £100 I think.
I've got one of the older 2hp Hondas, and found that the handles on the screw-in pads to secure the engine were too small for most outboard locks. There was, I recall, a specific Honda lock which took account of this, but it was very pricey.
I got over it by cutting off the original levers and replacing them with SS machine screws with largish washers. This provided sufficient size to ensure that the lock I had fitted.
This, of course, may be totally irrelevant to your newer engine.
Back in 2003 I purchase a TSM 005 lock from OML LTD...01326 318101. Total cost £60. The device is specific for a Honda 2.3 and is 5mm s/s tube which will keep a thief busy for ages with a very high security fitted lock which is very hard to pick. Worked so far.
"Honnlock" ... Bought mine on ebay for about £12. Wouldn't stop a determined thief, but would stop a casual chancer. Having seen what it consists of I would make my own. About 6" of 1" sq section dexian tube with a plastic bung in each end. Side 1 and 3 have 6mm holes in the centre to accomadate a long sterling brass padlock (screwfix) Side 2 has a 4" slot to accomadate the two thumb screw handles. Mine needed another 4 coats of paint.
I have a length of chain padlocked to the motor and to the outboard bracket.
It's stout chain and they are good padlocks. I've no doubt bolt cutters would have it off in seconds, but as the others say, it may discourage someone. Even proper locks seem to leave the threads of the clamp screws open between the pad and the bracket. A hacksaw blade or narrow bolt cutters in there would have it away.