Then my first suggestion is to just keep it simple - for example, purchase an inexpensive waterproof plastic match safe and fit it into the recess. Save a scrap of the stave that you cut off and whittle and sand it into a stoppper shaped somewhat like a champagne cork. If you want to get fancy with it, you could use a cut off screw or two thru the sides of the recess to act as a latch - the corresponding grooves in the stopper would resemble an inverted "L".

Personally, I'd probably spend too much time machining an aluminium alloy "cartridge" with threaded and o-ringed cap(s) to slip into a deeper hole etc etc.

If the dimensions otherwise suit you, in the USA a 2 AA Mini-Mag flashlight body and tailcap might be an inexpensive basis for a similar approach, although the body wall is a bit thicker than I would machine for myself. The "trick" to using a Min-Mag torch body will be figuring how to seal up the end that the reflector screws onto. It is NOT a common diameter-thread combination. What I suggest in this case is to reverse your paradigm: Remove the "head" of the torch and fashion a plug. The best approach for someone without an engine lathe would be to hand-file a scrap of aluminum to an easy fit (neither loose nor snug) and epoxy it in place. Better, use a small drill bit of suitable size and pin the plug with a steel pin or two (such as a shortened brad) in addition to the epoxy. Then simply use the tail cap of the torch as the lid to the container.

2 AA Mini-mags are typically available for under $8.00 US and many folks who regularly use them (like my family) usually have one or two around that have become "parts stores" due to long-forgotten batteries... you may be able to scrounge one for nothing and salvage it into what I described above.

OR - if you are a little clever and a little patient you could even conceal a small torch like a Mini-Mag in the top of your stave, complete with a decorative/protective matching wooden tophat.

I meant this note to be descriptive rather than prescriptive as you seem to be a clever and and resourceful fellow - hope this seeds some fruitful idea for you. If not, sally forth and we'll stir up some more ideas. I'm a bit hampered by my narrow scope of the sort of common hardware one finds in your neck of the woods - I communicate with some fellow hobbiests of the sort you term "model engineers" in your country, but we usually don't get into ordinary hardware comparisons.

Hope this helps.

Regards,

Tom