If you find a non locking card box you like, you may be able to just remove the closure peice it has and add a small hasp and lock. Something like this would work if you found a non locking on you like.
Locked card box is great though another way to not have any access to the box itself is having a sliding opening on the bottom. The centerpieces/floral ball rental place had a card box that wouldn't have any way to know the box was easy to get into unless it was turned onto its side to get to the bottom.
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Thank you Samantha! This is cute - though I wonder how reliable the DIY kits are sometimes 😅 I’m definitely going to look into it
Https://www.amazon.ca/Benjia-Wedding-Holder-Envelop-Reception/dp/B07V216654 I don't know if this is the style you're looking for, but it does come with a lock and is under $100. But I think yesterday you were looking for one that you can personalize, which I don't think you can do with this one unfortunately.