If I had to leave a trailer outside the "tongue lock" would be my last choice. Had a friend who lost the keys to his, just backed up to it and short chained it to get it home. Was only a few blocks but once it's out of your yard and in a garage good luck ever finding it. Anything that can be cut with a dremel or torch isn't worth having either.
I think the best idea is a multi-prong approach. Set a two inch solid bar into concrete and weld a ball to the top then attach the trailer to that, make sure to weld a plate on the bottom so it can not pull out of the concrete and is at least three feet in depth. Put wheel locks on the wheels, at least two different styles and then make a solid hasp of at least 1 inch diameter rod that can run through both sets of wheels and be secured on the other end to prevent the trailer from moving, passing this hasp over a leaf spring is an added bonus. Have another bar with a heavy loop welded to it sunk in the ground and an extremely heavy chain wrapped around the axle and the hasp made to go through the wheels. Finally make sure the trailer is loaded with so much undesired junk that it will take a person an hour to unload it and it's too much to even move the trailer with in the first place.
Otherwise remove the axle and wheels, make a detachable tongue and remove it and flip the trailer upside down when not in use. No axle to even mount the wheels on, no tongue to pull with and a pita to even deal with unless your going to load it on to another trailer. Just have to fabricate some stuff so it can be easily put back together when you have the stuff.
I think the best idea is a multi-prong approach. Set a two inch solid bar into concrete and weld a ball to the top then attach the trailer to that, make sure to weld a plate on the bottom so it can not pull out of the concrete and is at least three feet in depth. Put wheel locks on the wheels, at least two different styles and then make a solid hasp of at least 1 inch diameter rod that can run through both sets of wheels and be secured on the other end to prevent the trailer from moving, passing this hasp over a leaf spring is an added bonus. Have another bar with a heavy loop welded to it sunk in the ground and an extremely heavy chain wrapped around the axle and the hasp made to go through the wheels. Finally make sure the trailer is loaded with so much undesired junk that it will take a person an hour to unload it and it's too much to even move the trailer with in the first place.
Otherwise remove the axle and wheels, make a detachable tongue and remove it and flip the trailer upside down when not in use. No axle to even mount the wheels on, no tongue to pull with and a pita to even deal with unless your going to load it on to another trailer. Just have to fabricate some stuff so it can be easily put back together when you have the stuff.