After a bit of looking I came across the Master Receiver Lock 1473DAT
at my local auto parts store that I thought I could make work as a fork lock. The package included 1/2" and 5/8" receiver pins and a single lock.
Here's the 5/8" pin and lock.
I used an abrasive cut-off wheel
to cut the pin so that the fork upper would hit the lock squarely when locked on. I ground off some of the chrome plating near the end of the pin then welded it onto the frame head tube so that there would only be a few degrees of fork movement.
Here's how it looks when locked. Its not super secure, but will prevent an opportunistic roll away theft.
I can also lock it in the other direction, but it is harder to access the lock and there's about 30 degrees fork movement.