A few things to think about here – protecting yourself against the manufacturer(s), and then protecting yourself against the rest of the world 🙂
The most common way to cover yourself against a manufacturer would be to have an NDA (non-disclosure agreement) signed off – so that the manufacturer confirms that they have no rights to what you are showing them, and that they won’t copy/mimic for either themselves or another party. Here’s a link to the Kindrik template for an NDA: https://kindrik.co.nz/templates/mutual-confidentiality-agreement/ I’d strongly recommend you contact Averill Dickson to tweak the NDA so it fits your purpose.
Protecting yourself against third parties once you’re in the market is a bit more tricky – depending on the nature of the product, your options are patents ($$$$), registered designs ($$) or relying on copyright where you can (not much $).
You really would need to sit down and get a bit of advice around this before you market/sell the product. Happy to have a chat to you in order to steer you in the right direction 🙂