What is the most inexpensive way to protect the design of my product?

Hi, you can evaluate either to do design registration on a cost and benefit base. Doing design patent registration and bringing to a lawsuit causes a cost. when we take those as negative and profitable effect of the new designs' gains as positive, if the positive difference is meaningful, then it is satisfactive to do those registrations. I don't know the quantity of your designs but i can suggest a way which is cheaper at start but may be more in total. You can do multiple design filings in another country like European. Hague System may be helpful. It is going to give you a priority date before filing in USA. You have to work hard after filing of this application to estimate the reaction of the buyers to your new designs. when you know which designs are appropriate for the market (sure it must be before the end of priority date), then, you can do design patent filings using your priority claim. Your time for this is going to be 6 months.