Box knife is the way to go. A straight edge with a backing material under the cardboard like another piece of cardboard. Hot glue gun works wonders for joints. A little dab will do ya.
I used to build molds for fuel tanks we would use high grade cardboard and 3/4” particle board strips for rigidity. We used staple gun, glue gun, masking tape. After a mold was built it would be wrapped in masking tape then wrapped in fiberglass cloth. The cloth would be seamed with a wood burner “kinda melted together. It was coated with polyurethane and spun on a rotisserie. The next day a hole was cut for the pump and gas fill flange. It was filled with water then soaked for an hour and the cardboard and wood would soften so you could break it down and remove it.
it got filled with fuel resistant foam blocks, a pump and flanges then placed im an aluminum can (tank). Racing cells that would not explode on impact, they could catch fire but not explode.
Cardboard is an art substrate that is undervalued. You can take a dull Phillips head screwdriver and run it along the corrugated lines to score it for perfect bends. Cross scoring against the grain requires a straight edge and a dull flat head screw driver.
I know TMI!