Can you help me brainstorm security measures?

I work as a street musician. I have only played in safe towns, and I am getting ready to play in a city for the first time. My accordion lives in a large wooden box that collects tips while I am playing. I am worried that someone will run up and grab my box (like in the movie “Once,” my fave). Also, I will have a backpack with me that I would prefer to not have stolen, even though I won’t be able to wear it/hold it while I am playing.

I have already made a cardboard screen to cover the box with a slit for tips, so I think I’ve prevented bad guys just grabbing bills. I’m still worried, however, about the entire box, and my backpack.

I’m envisioning installing a big eye bolt in the box then chaining it, going through my backpack, to a parking meter or something. If there’s nothing to attach it to, maybe my leg, although that’s not ideal.

Can you think of anything? Any suggestions are much appreciated! Thanks!