The weak point is usually the sheetmetal. You usually need to reinforce the hole with large washers (called Fender Washers) or a piece of steel plate.
Another great way is to drill small holes and then weld the reinforcement plate to the strong box through the holes No bolts to undo, and no loss of space in the strongbox. You can even use the holes to run a light into the box.
Frankly, in the trunk is usually not a great idea. Too much space to insert a pry bar. I prefer to install the strongbox in a location that the content of the vehicle makes it hard to access. Like under a seat where the seat has to be removed to get the box in or out. There are even some replacements or inserts for consoles and glove boxes that allow the factory door to be used as concealment.
I even saw an in-dash DIN size strongbox with a fake radio as the front panel. You tapped in the 'code' on the preset buttons to open. Little small for a full size pistol, but such stuff exists.