I have a '60 model 261 truck and would like to learn of a simple and 
unobtrusive way to secure the cab short of a hasp and padlock or 
mongrel sentry.
                - - - - - -

when i got my 59 it had no keys for the doors.   I took the handles out and
took them to a locksmith,   who charged around $15 each lock to make a key.
  this is the easist.

or, you can try to get a key cut the the key code, which is on the lock (like
3Z4 or SU345)

or take the lock apart to fit a key.

does the ignition key fit the door handle?