That’s really the only advice I can give on models and copyrights.
Nevertheless, here are some examples of contract language that may be illuminating. Bear in mind that I AM NOT A LAWYER AND THIS IS NOT LEGAL ADVICE. I provide no warranty of any kind and assume no liability for your use or misuse of these examples. There are lots of deadly details, regional differences, and variations in opinion about good contract terms. Also, these terms have been slightly adapted to conceal their origins, which may have unintended consequences. Get an IP lawyer to review your plans before proceeding.