The egg creates get mold and they start to fall apart as they suck up the moister, so that not good for your spiders in the long run, Unless you paint the egg boxes with resin this will seal them and still give you the hides that you like.

You can get those small pot plants holders and put them in, you also get small hides from pet stores.
Or pieces of bark.

Well as mom in law bakes, we have many many egg boxes. We were planning on using the egg boxes as temporary shelters till they got bigger and we could put in something nicer or more natural