There was a post about a year ago on this subject of locking gas caps for the TN series; myself included.
Someone had a part number for a locking gas cap; manufactured by NH... I believe. If memory serves me right, the gas cap was for another NH tractor, or skid steer....or similar machine.
Wish I would have saved the post; with fuel prices going up like crazy, it would pay to have a locking gas cap...even to prevent vandalism.
If you find the part number, please let us know.
Thanks..CL