Here are some suggestions from safefirearmsstorage.org
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Trigger Lock
A trigger lock can be used to keep someone from being able to pull the trigger. Trigger locks are available in a variety of configurations, from basic key lock options to those that require combinations to open the lock.
Cable Lock
The cable lock can be used on most firearms. The cable is most often placed through the gun barrel, which generally impedes loading and firing.
Storage Case
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Storage cases come in various sizes and can be used for both storing and locking up a firearm. The case can be locked, but the lock must often be purchased separately.
Lock Box
A lock box is like a small safe that allows you to store your firearm safely and securely via key or combination lock or by a digital keypad, to prevent unauthorized access to the firearm.
Locking Safe
This safe comes in a number of sizes to accommodate long guns, such as rifles and shotguns, and includes spaces for storing handguns as well. Much like the lock box, the gun safe has the option of a combination lock or digital keypad to lock it.