First off, work out exactly where the tunnel will go and use a sheet of paper to mark out the shape you want...
Next, cut out the template and use it to mark out the tunnel roof...
I used hardboard from the back of an old flatpack wardrobe (I never throw any wood away!). It's so thin that it can be cut using a Stanley knife.
The supports are made from bonfire night rocket sticks found in the street and nailed n' glued to the top. The front is made from heavy card and covered with good old Metcalf brick sheets.
After placing all the bits of scenic stuff to hide all the faults, it looks OK.
