you can make an armory… concrete walls metal outside metal inside with wooden floors everything in one once you subdivide room if you want the door to be non vault and nice looking get a http://surcelikkapi.com/en/index.php
make sure you have ventilation as a sealed room with vault door can suffocate you
failing all of that you can get a set of old government surplus lockers or filing cabinets…yeah the old stuff from 50 and drill inserts into the floor put a good lock on it so you cant open crappy lock with a piece of pop can or bolt cutter. we welded a job box housing over a new lock loop total cost $5 and 1 hour of work
you can make steel magnetic lock boxes and cut them into car seats especially ones that fold and you dont use to transport people so you dont violate any “safety” protocols have them open with a fob key and a hidden button. most thieves wont tear up car seats that is something personal not just for the sake of money. even if they find box they will have to unmounted it and cut through top when magnetic locks engage all the way around there is no way to open them making the box out of 1/2 inch steel pretty much precludes an easy cut want to be a real bastard carbon fiber the outside grinding wheels and torches will become a major pain or destroy whats inside.
bolt through the frame but use another plate on backside so if they try to pry it they will have to pry through a plate same size as the box with 4 heavy bolts… most will give up before that. weld the heads and bolts to the plate and plate to the frame. 2 day job
for super cheap method