Elias built the frame out of two slats of wood, then drilled holes in the top to make a curved roof out of wire. He also wove some string perpendicular to the wires.

Next, he made a form out of cardboard and poured in some craft cement mixed with gravel. 

We didn't have quite enough cement, so we used plaster to fill in the holes and ended up covering the whole thing again to give it an even surface.

Next, we painted the tunnel an exciting gray.

And finished!

Tunnel in action!