Reddit reviews Hangar 9 Fiberglass Cloth, 1/2 Oz 1 Sq Yard

Ryonet has a number printing system that they talk about in a few videos.

Since you probably can't expose the boxes to ~400 degree heat, you probably want a water-based ink, and let it air dry.

I think that surf- and snowboards have the designs printed onto sheets which are embedded into the top surface with resin. You could print onto the boxes and then paint over it with a finishing resin to protect the design. Or possibly some sort of marine clear coat spray.

Or... if you wanted the design inlayed, print onto fiberglass cloth and bond it to the box with resin adhesive and finishing resin.