Get some nice locking boxes and combination locks that can sit in each other like matryoshka dolls.
Would you happen to remember where you bought these boxes? The ones I find are always either really expensive or really cheap (too cheap to be worth getting for school activity use, I have students who would definitely end up accidentally destroying them). That middle ground has been really elusive for me.
What I did was buy some extra large, large, and medium sized hinged tupperware containers, drilled some holes opposite the hinge and added a bit of cheap chain and combination locks, which I bought from daiso. If you cant find hinged tupperwares, just make 2 small holes on one side of the container and another 2 on the lid and wire them together for a makeshift hinge. If the container isn't frosted like the ones I had, just, I dunno, spray-paint them, or line the inside with some paper. Daiso is your friend!