I don't have a trailer but I did mount one in my motorhome. I bolted it to the metal framework for one of the dinette seats.
Got it at Staples for $99. It's not waterproof or fireproof rated, but it does have a 1 hour dirtbag proof rating.
If someone wants in bad enough they'll get into anything, but I'm a retired LEO and I don't go anywhere without a pistol. If we're visiting someplace where I can't carry it then I have a secure location to store it in the motorhome while we're away. It's also a good spot for storing spare cash, passports and things like that when we're on the road.