It would be nice it the Handy and Memory Chests would lock themselves if a Private Memory Card was installed so that 1) That card cannot be removed, 2) The Chest itself could not be broken. Optionally locking the chest from opening if all Memory Cards were marked as private.

Well the Memory Chest doesn't use cards... But should I add a Private/Public mode to them too, which would simply mean they wouldn't allow any access in Private mode except for the owner manually opening them?

I'm not entirely sure from the top of my head if the chests can be made conditionally unbreakable, but I can look into it, it may be possible, at least when a player is manually trying to break them.

Would the Handy Chest not opening if all cards are private have any real benefits? If the chest can't be broken and the Private card(s) not taken out anyway, I don't really see the benefit? Or would the chest also rather have a master Public/Private mode? Just wondering what would be the most flexible option for things that somebody may want to do.

Edit:
I'm thinking a master Public/Private mode really isn't that useful of an option for the Handy Chest? Why would you make the chest private and use Public mode cards, instead of the opposite. Because using Private cards for limiting access would still allow using Public cards too in the same chest, to allow access to certain items but not others.

Okay here is another thought: What if the memory cards in a Handy Chest could simply be "locked in place", maybe by middle clicking on the selection buttons. So you could even lock Public cards so that the cards themselves can't be taken out? And then if any of the cards has been locked in place, the chest would become unbreakable?

A private public mode on Memory Chests might be helpful and yes I'd keep it simple owner only, a more complicated system can always be developed later if warranted.

I know I can break protect chests using the Protection Wand from Not Enough Wands, and toggle it off again. So it should be possible to use the same method. I'd assume it simply doesn't send the block break event to the server or the like.

I suppose in hindsight locking the Handy Chest entirely would only really have cosmetic value, making it obviously that that chest is locked to all, but not a needed feature by any means given the rest.

As for locking cards in place and break protecting the chest that sounds like the best solution by far.

I'm thinking that being able to lock any cards in place would also be the easiest way to implement this.

One question though: Who can toggle that locked state? The owner of the card, or the owner of the chest? Or should it be the owner of the card, but the chest's owner can still override that?

I'd say the owner of the card. Is there currently a way to transfer ownership of a card to another player?

If the owner of the chest can't toggle the mode on cards they don't own, could they still break the chest if there are locked cards? Or would it simply be that each card owner has to unlock their card before anyone (even the chest's owner) can break the chest?

And no, I'm pretty sure currently the owner is permanently set to the one who toggles the privacy mode the first time on any item that supports it. I could add maybe sneak + right clicking to remove the owner data from module items (only when the owner does it, in either public or private mode).

The latter, if anything is locked then the chest cannot be broken. This way owners of their own cards could remove them and put them in a bag or a different chest.

Sneak + right click to clear the owner sounds like a good method, then a new owner can set the card to themselves.