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A liability release would be a good idea if your husband is planning on selling many of these boxes. The release would state something to the effect that the boxes are made for exercise (or whatever they are made for) and that even using the box as intended may result in injury. Buyer assumes all liability for any injury that occurs as a result of using the box, etc.
If your husband is going to sell a lot of these, it would be a good idea to start a limited liability company (LLC). This will provide your personal assets some liability protection. If you go this far, it would be a great idea to get some liability insurance.
I hope this as been helpful. If you need further information, please let me know.