AFAIK you cannot trademark a product design, that is what patents are for.
But you can register a trademark for your logo and incorporate your logo physically into the product, then it is either identifiable as a knockoff (no trade mark logo) or trademark infringement if they use the mark.
"A trademark is a recognizable sign, design, or expression which identifies products or services of a particular source from those of others… The trademark owner can be an individual, business organization, or any legal entity. A trademark may be located on a package, a label, a voucher, or on the product itself. "
If your item is made in the USA, google the Made in USA program to see if you qualify. Using this mark gives you additional protections to block imports and fakes.
In your Amazon listing, include as many Product Specific (not seller specific) details as you can to differentiate your product; material, thickness, other physical attributes, where it is made, etc.
You also have copyright on the design and wording of your packaging - make sure your company name and a copyright statement is on each item of packaging, instructions, etc.