A strong non-disclosure agreement (NDA), and a US based manufacturer are your best ways to protect your intellectual property if you’re working with a third party manufacturer.
Too often you hear stories of foreign manufacturers making a product for their client, then slightly modifying the molds and making essentially identical products for their own resale.
A good NDA will state that they can use the molds only for your product, and they cannot use any of your proprietary designs or information for any other purpose other than making your products.
Choosing a US-based manufacturer will make it easier to enforce the NDA, easier to prevent them from further misuse of your IP, and easier to obtain damages from their breach of the NDA. It can be difficult and expensive to drag foreign companies into court in the US for similar purposes.