Hi Becca,
You can make hide boxes from lots of things. Some people use empty toilet paper and paper towel rolls. You can cut them in half lengthwise and get twice the bang for your "buck. Some people use empty tissue boxes. I use upside-down, clean, empty flowerpots with a "door" chipped out of the bottom edge. Also, lots of people use plastic water dishes that are hollow and the snakes hide under them. As for size, you want your snake to feel like it's hiding, so you don't want it to be huge compared to the snake. Your hatchling should like half a toilet paper roll just fine. The pots I use are the 4 inch variety and my snakes have liked them since they were tiny and still like them now when they're about 24 inches long. There are also lots of things for sale at pet stores, but they're more expensive.
As for the aspen, it depends on the size of the bag. You can get a huge bag of aspen in the small pet section of petsmart that will last you many substrate changes. I don't remember how much it costs, but I believe it's under $15.
Welcome to the site. :wavey: