So when I was much younger (like around preteen) I had a large wooden "toy box", this was basically just a chipboard box about 4-5 foot long, about 1.5 foot front to back and about 1.5 foot deep, the top of it was hinged to provide a lid and I had 2 made to fit cushions on top so that it could be used as a seat when I had friends over.
when I started getting nappies regularly I took some of the old toys out and stored them in there, since most of the time it was being sat on most people wouldn't think to open it and it was older toys and things in there anyway, I could cover the nappies and unless you went digging you could open it and not immediately see them.
At some point soon after I fitted one of these types of locking hinge latches to the lid.
They are dirt cheap (about £3 at the time) and easy to screw in and while they could probably be easily removed or broken by anyone who really wanted to get into the box or simply ripped out of the wood they did the job of preventing nosy snoopers from simply having a look at what's inside, so for instance if my mum had come in to clean or put stuff away she wouldn't be able to open it or if a friend got bored and wanted to look around they couldn't.
If anyone asks it's just some of my old keepsakes that I want to keep safe (my mum isn't the type to demand I have everything open to her at all times or wanting to remove any sense of privacy so I had no worries there).
At some point around my early 20's a friend of mine helped me build my own computer desk, complete custom build to fit the space and with all the features I needed (space for the pc/monitor/chair, shelves for games and discs etc. one side of it is a fairly large cupboard that has a lock on it, again a very cheap little thing that's less about security and more there to keep the door closed and not swinging open freely but it does so with a key so that became my storage for a while.
Nowadays that cupboard is full of other AB things like bottles/clothes/sippies/dummies etc. as well as more "adult" toys and the toybox really is full of old keepsakes and standing on end at the foot of my bed like a little table (I have a cabin style bed) and I am much more confident about things with less need to hide the nappies since my mum knows I wear, so I simply keep a bunch of loose plain ones in my drawers ready to use with my bulkier cloth stash in an unlocked cupboard on the bed itself so they are out of sight.
I do however keep unopened or part used disposable packs and any printed/AB nappies in a special hidden place as my mum may know I wear but only as a DL/stress relief not any AB things (which really are a very tiny part of it for me anyway). That special place is an old armchair that I have in my room.
We used to have it downstairs and my mum was throwing it out a looong time ago when she got new furniture, I wanted a decent comfy chair in my room anyway so I kept it. The fabric on the very bottom was ripped off a long time before so you can access the inside of the chair and the material that covers the springs has also since come off so what I have is a piece of fibreboard (MDF) that has been cut to the perfect size to slide onto the chair over the springs with a cushion on top so it still works and looks like any old armchair.
I can tip the chair backwards and from underneath I found I can fit in about 3 packs of nappies inside below the springs, I then put the chair back down and you can't see them, it I want a nappy out from it I lift up the board and cushion and can take out a nappy from a pack through the springs. A perfect system.