I've made my own. Cool site called templatemaker: https://www.templatemaker.nl/en/ you can plunk in custom dimensions and create a template. If you have a flatbed printer, you can print a cool design on one side of cardboard, and print the template on the back - hand cut and score to fold. I used this to create substrate sample boxes for our wholesale customers.
Edit to say this works well if you only need a dozen or so. If you get into hundreds or thousands - you won't want to make them yourself with out a plotter and folder - then you really do want a packaging company.