Creating a custom openable/closable and/or lockable item, or adding that support to an existing item, has become very easy with Expansion 1.6.53.

Simple examples

An example for a container item that can be opened/closed and will allow access to its inventory accordingly (we will be using an existing item as a base, the vanilla SeaChest, note that this also means there will be no open/close animation because the SeaChest doesn't have them):

config.cpp

class CfgVehicles
{
	class SeaChest;
	class ExpansionOpenableChest: SeaChest
	{
		expansionIsOpenable = 1;
	};
};

ExpansionOpenableChest.c

class ExpansionOpenableChest: SeaChest {};  //! Only the class needs to exist for the functionality to work

---

Another example using the SeaChest as base class, this time making the item lockable with the Expansion codelock:

config.cpp

class CfgVehicles
{
	class SeaChest;
	class ExpansionLockableChest: SeaChest
	{
		//! @note Att_CominationLock will also work if you want to support RoomService's CodeLock mod
		//! as well, but then you should disable the vanilla CombinationLocks from spawning
		attachments[] = {"Att_ExpansionCodeLock"};
	};
};

ExpansionLockableChest.c

class ExpansionLockableChest: SeaChest {};  //! Only the class needs to exist for the functionality to work

---

And finally, a chest that can be opened/closed and is lockable with the Expansion codelock:

config.cpp

class CfgVehicles
{
	class SeaChest;
	class ExpansionOpenableLockableChest: SeaChest
	{
		//! @note Att_CominationLock will also work if you want to support RoomService's CodeLock mod
		//! as well, but then you should disable the vanilla CombinationLocks from spawning
		attachments[] = {"Att_ExpansionCodeLock"};
		expansionIsOpenable = 1;
	};
};

ExpansionOpenableLockableChest.c

class ExpansionOpenableLockableChest: SeaChest {};  //! Only the class needs to exist for the functionality to work

Note that all three examples exist in Expansion as spawnable items.

As you can see, the scripting required is minimal. You can of course still add (e.g.) your custom animations when opening/closing in script.

Advanced scripting

If you want certain actions to be only available when a player is aiming at a specific part of your custom item, you can use scripting to check for specific selections.

class MyCustomOpenableLockableContainer: Container_Base
{
#ifdef EXPANSIONMODBASEBUILDING
    override bool ExpansionIsOpenable(string selection)
    {
        return selection == "my_custom_lid_selection";  //! Can only open/close if looking at lid
    }

    override bool ExpansionHasCodeLock(string selection)
    {
        if (!super.ExpansionHasCodeLock(selection))
            return false;
        return selection == "my_custom_codelock_selection";  //! Can only interact with codelock if looking at it
    }
#else
    //! If Expansion isn't loaded, you'll have to implement opening/closing/locking yourself
#endif
}

The #ifdef is there so you can make your modded item work without Expansion as well.