This is what I use. Master Lock - 6 Trailer Locks Keyed Alike.
Total trailer security Keyed Alike - One key opens all six locks. Each lock comes with two keys, for a total of twelve keys. Multiple 6KA trailer lock packages CAN be keyed alike. If you ordered a quantity of 2, 3 etc we would automatically key them all the same. Latch lock doesn't fit some 2-5/16" couplers. See description below for latch lock (#37) applications. For 2-5/16 couplers the better lock set is: Master Lock - 6 Trailer Locks Keyed Alike #6KA-3796230-3. Three (3) high security 6230KA padlocks. Coupler lock fits 1-7/8", 2", & 2-5/16" couplers. See description below for latch lock (#37) applications.