I think there are several ways:

Method 1: The work belongs to the designer. It is recommended that the designer applies for copyright for the original work;

Method 2 : When communicating with customers, inform them that applying for copyright is a way to protect their intangible assets. Let customers not only come to you to design works, but also apply for copyright business; this way you will also earn more;

Method 3: If customers on their platform do not adopt your work, they cannot download the original and can only see a sample. To be honest, the cost of plagiarism is very high; if a dispute arises, as long as original evidence can be provided, the liability can be traced