Storage box - keeping bugs out

We have a large storage box on our patio for chair cushions. One corner of the top has started to curl pretty bad. We put a heavy pot on it and left it for 6 months to see if that would correct it - it didn’t. There is almost zero chance of rain getting in the box, but we have a lot of woods behind our yard and I am concerned about spiders, etc. Any suggestions of what I can put in the box to repel them that won’t stain or stink up the patio chair cushions?

Thanks!