It helps to know your commercialisation ambitions. Basically three questions:
1. Who would sell this? Your business, or do you want to persuade some other business to take it on.
2. Who would make it? Depends a lot on the answer to (1).
3. Once it starts selling, is it at risk from copycat versions? Commercially, I mean - I've improvised devices for a particular aspect of making musical instruments which would not be worth copying, no matter how clever they were (they weren't), because the market is minuscule. If so, taking steps to keep your device confidential (via non-disclosure and use agreements) is an important first step, and then you need to think about financing the process of protecting your intellectual property rights (this runs from tens of thousands upwards, depending on the device). Financing probably helps answer (1).
If, let's say, your device will revolutionise shelf brackets, making 50% of what's out there redundant, then it's a big bucks project and needs big bucks financing. Your prototype maker needs to be someone you trust - if a local small shop makes it for you, and some employee sells the idea, they don't have the assets to compensate you no matter what agreements they signed. Plus, the brand name you approach might well refuse to sign a confidentiality agreement in case they are currently working on something similar. Navigating the big bucks financing maze is tricky!
OTOH, if it's a niche market device which would make a nice small business for you, you might be better off retaining your market by making it better/more attractive/cheaper than the potential competition. IP rights protection might just be too expensive to worry about. In which case, the small engineering shop down the road is where to go for your prototype.