Step 5: Get some Glue
Wheatpaste: At less than a buck a gallon, this is what I will be using. You can view my wheatpaste instructable here, or basically heat 1:4 part flour/water until it get's thick. Wheatpaste, when made properly and applied correctly will be pretty much as strong as the glue that holds the corrugations of the cardboard together (they use a starch glue, white flour is starch...). It's used by paper machers and also graffiti artists to post paper pictures to concrete walls and create a rock hard irremovable poster. I would recommend adding any bacteria deterrents you have (see instructable).
1:1 Wood Glue: You might be able to find a gallon of wood glue at your local hardware store for about $10 if you're lucky. This should be diluted with water 1:1 because we're covering such a large area, and only want a thin coat, and also to help the glue soak into the cardboard. This comes out to $5 per gallon. The advantages of this is it's much stronger, the corrugations will always rip before the wood glue seams, and is easier to make and apply (but not much). After doing this with wheatpaste, I would recommend this way, simply because the wheatpaste is not as sticky as it needs to be.
Note: You could also use Wallpaper paste I guess; it comes in a powder at the hardware store.
You will need a minimum 2 gallons of either to properly glue the amount of cardboard I am doing.