Mass production contracts

One way would be to buy several thousand units up front.

Other than that, determine the key process(es) that would be involved in manufacturing the product. Search out some manufacturers that produce similar items, and contact them.

Be prepared to accept suggestions/modifications that make the item easier to manufacture. Simple geometry, commonality of parts, conventional fasteners, etc.

Since nobody knows what this is, only vague suggestions can be offered.

"How can I get companies to invest in mass production of a prototype that I have already built", without giving it away, or having it stolen? Do you have intellectual property rights to this thing? Do you make potential "partners" sign a non-disclosure agreement?

And, let's put this into perspective. How many parts in each "gizmo"? How complex? materials? Tooling costs?

Do you have a market for these things? A distribution network? A business plan? Investors? Any money yourself?

The more you have to offer a potential partner, the better your chances are.

Your question lacks substance that would indicate that you have anything to offer anyone, other than a hand built prototype.

Good luck.

Write a business plan, and convince the potential manufacturer that you have done the market research that shows that your product will sell. Offer them a license to build the product. They make the profit on the item, you get a royalty. Just be sure that you've protected your intellectual property.