Cardboard is widely used to mock up structural designs or to make templates of other materials that are either too expensive or difficult to work with. Great stuff!
I have a brother who watches my grow and decided to do it himself. I use 2' x 4' pieces af cardboard with mylar glued to them used as reflectors. I hang them or just prop them around the plants. My bro decided to build a wooden veg box and glued his mylar in it. Well, it looked really nice and it worked, but when he decided to make changes to his design the wooden box and glued mylar were worthless to him now. "I should have used cardboard." I can start a grow in a smaller area using these reflectors and expand the area in a minute. It also alows me better access to the plants. Bro now has an area in a room with an 8' x 6' area 'tented' with panda film. He now uses mylar on cardboard to reflect light close to the plants. Instead of stray light traveling from lamp > panda > plant (6'+ total distance) it travels a total of 2' -3'. At least four times the amount of light hitting the plant.

And for a total enclosure, as you've built, cardboard works wonderfully. I also think that in the event of the unthinkable, my grow (of 6-10 seedlings/plants) is discovered by authorities, it doesn't look very 'commercial'. Definetly looks ghetto and it can be taked down and put away in short order. I have yeilded up to one pound of dry bud from a one square meter area (aprox. 4' x 3').

Endless other possibilities using CB for inside growing.