A shed box really isn't needed. If you keep a water bowl in the enclosure, you should be fine. Leaving the snake for 3 weeks shouldn't be a problem other than for fresh water. It will shed and be fine. But another issue could be more of a problem for you instead of the snake.
Many times, once the snake sheds, it will poo almost immediately after. That poo, if it happens once you leave, that poo will sit a stew until you return. That could cause a problem with the smell brewing for 3 weeks until your return. Walking into the front door of your house, you could be hit with a very bad smell.
Good Luck!
Jim Smith
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