Without a patent, copyright, or trademark, protecting intellectual property can be tough. The threat of a lawsuit might work, but that involves time and money, and can be a big hassle, which is why a lot of crooked people steal the IP of others- because they know that few people will actually go through the trouble to sue.
If a knife maker steals your design, unless they are making big money from it, it is highly unlikely that you will find any lawyer willing to sue on your behalf for a commission. Civil lawyers are typically only interested in suing "deep pockets" and going after big paydays.
However, in regards to trying to protect your intellectual property, one way I've heard of people trying to protect their designs without going through the lengthy, expensive legal process (patents, copyrights, trademarks) was to mail a copy of their designs to themselves or a family member using a priority mail envelope, but keep it unopened. When you mail something it gets dated by the post office. And if you ever want to try and prove in court that you came up with the design you can present the unopened, dated envelope to the judge, open it in court, and prove that you at least had the design prior to the date on the envelope. This of course offers no guarantees, but it's something.
Another thing you need to do is keep perfect records of all communications with the maker, preferably emails. This way you can prove that you had a deal with the maker to produce your design. And if you talk on the phone, keep a log of the calls with dates, times, and a detailed description of what was discussed.
I've seen where some people think that taking pictures of their designs on a newspaper with the date on the newspaper showing will serve to document the age of the designs, but that won't work because a person could always get ahold of an old paper to take a picture.
