How do I reinforce cardboard without warping it?

Question

I'm working on a project where I'm building an 11" by 11" wind tunnel out of cardboard, duct tape, and a 22" box fan. I'll need to reinforce the cardboard somehow, and I've come to the conclusion that varnishing or Mod-podgeing it would be best, although I've also found out that varnishing cardboard can warp it. I need as straight and flat panels as I can get. I most likely will not be varnishing the interior of the tunnel. How would I go about varnishing/mod-podgeing the cardboard without the annoying warping?

Answer 1

Varnish both sides and that should avoid the warping.

Answer 2

It would be a lot stronger if you glued cardboard strips to the outside, perpendicular to the panels.  Or, take pieces of 14 x14 cardboard, cut 11 x 11 holes in them and slip them over the outside of the tube.  Glue them or duct tape them in place.

Answer 3

try ribbing the cardboard. apparently if you are allowed to use varnish, then you can use other types of adhesive, or, you could try a stronger cardboard. ask around your local grocery stores and hardware stores, try double or even triple corrugated cardboard. oh, and I understand the whole venturi effect, but that box fan may not be able to create the pressure needed to make the venturi work, if this is so, you could always try using a second fan. two strapped together, or one on either end, box fans are cheap. if you find that a box fan just can't get it done, look for a high velocity fan with metal blades. they always give you the most power.