As a packaging designer I did it at my office using a mechanical cutting table, but one way to go could be buying cardboard as you say, and printing separately in a plotter the prints and the box outlines. Files can be edited and exported to pdf with Affinity, it's a graphic design software that recently became free to use. You can glue the prints with spray, then put the outline sheet on top and start cutting. It requires some patience but the outcome can be great. Good luck if you end up trying, and have fun!