I am just wondering if there are any small locking boxes can be found that can locked with a combination lock that is not actually built in to the actual panel that closes the box (IE like with a safe). What I mean is something like a small hole which aligns with a part of the box to secure it shut. I have attached an example of the layout. Essentially the item in the image would be about 20 centimetres wide at the most and non see through. I didn't expect something so simple to be so incredibly difficult to find. The attached image gives an indication of roughly what I'm looking for. I did try with a mini safe recently, but unfortunately I am a bit vulnerable to making oversights, which meant that all the keys ended up being stuck in the safe that couldn't be unlocked. But fortunately it had small ventilation like holes that I was able to enlarge using a pair of pliers and scissors to recover at least one of the keys.
Call me sleep deprived, but I don't understand why you're looking for this over a small combo safe? Ease of using bolt cutters?
You can get hinged boxes at a craft store that has a latch on it or buy a latch at a hardware store...Create your own....I have set up in another frozen key game.
If you already have the box, or find a suitable one without the fitting then you could DIY and screw or rivet a hasp to the box. Searching for "furniture hasp" or "cabinet hasp". eg: https://www.ebay.co.uk/p/Solid-Bras...h-Strap-Lock-38mm/2254452826?iid=191395795699