Hmmm, I think it is easy to convince them.
1.) Pay for their research into developing the Kit.
2.) Pay for the molds being made.
3.) Pay the factory to take another kit of the production line to produce yours.
4.) Offer to buy any stock from the production run that didn't sell.
Might set you back a few x00.000 USD though.
Unfortunately ANY company will not listen to a few but will always produce to the masses, for the few there are always the Garage-Kit companies and similar.
Most companies will also ONLY do research within their own borders/country as that is their main customer market.