would be good for boxes too Making Felt Bowls . "While it is simple for anyone to make a felted bowl, it is challenging to make one well." quoting Maria Roth, the author of linked article and maker of these bowls
Secrets of wool-felting masters (the entire site is an amazing resource for all things felt! Lots of inspiration there!) Please note safety issues.use a circuit breaker, minimal water, waterproof barriers or better still go cordless!