This is a great question. I understand that I am late in answering but I will do my best. In the business world you can never guarantee anything. In my experience there are two things you can do.

One find a supplier/designer/manufacturer that you trust. One that wants to establish a good pipeline of potential customers and stealing ideas would detract business. This is a little harder and would require good research and maybe searching for designers that cost a little more. But it will be worth it in the long run if they are trustworthy.

Two, create a non-disclosure agreement. This will allow that whatever you and the other party discuss is considered a private discussion and not a public disclosure. For the patent, this will allow your design to still be considered undisclosed and thus, still eligible for a patent.

Hopefully, this will give you a strong start in your new product. Good luck! If you need any help in design or CAD modeling I would be happy to help, please know that I am available to assist and I have experience in this!