A trailer hitch lock is designed to keep the trailer hitched and attached to the towing vehicle. The lock serves a dual purpose: an extra level at attachment between rigs, and a safety precaution to prevent the theft of the trailer and/or the towing vehicle. A homemade trailer lock does not need to be a complex and intricate affair. Use an extra long padlock and power tools to drill out an extra hole to be used as the securing area between the two rigs.
Park the trailer and place wheel chocks under the tires. Keep the trailer attached to the receiver hitch on the towing vehicle.
Mark one side of the connected hitch in a spot where the two pieces are nested into each other. Mark the centre of the trailer tow-arm. Place a 1/2 inch long metal bit (as long as the trailer hitch is wide) into the power drill and bore out a hole at the marked location. Make the hole go through all walls of the trailer hitch --- the trailer side and the tow hitch receiver.
File down the sides of the bored out hole to smooth the surface.
Insert the extra long padlock through the hole and lock the lock clamp. Position the lock to make it difficult for potential thieves to access.