This tortoise shell was built for our daughter's youth theater's production of Sleeping Beauty. It is made from cardboard and covered in fiberglass.
Step 1: Getting started
I found an image on the internet from which to model the shell. I took measurements of the actress from the top of her next down to about four inches above the back of her knee. It was about 34 inches. I figured a good proportion of width to height would be about 2/3 so I measured a 34" x 22" rectangle on a piece of cardboard. The shape was drawn freehand. I simply rounded the front corners and then tapered the back to make it wider in the front by about an inch. To make the rounded shape, we determined that about six inches would be enough to give dimension without being too cumbersome to wear. I cut a 6" wide piece of cardboard and curved the front and back to make the shape of the shell. The front scooped down and then out to make the collar area.
Step 2: Building up the shell
I used the same technique of building the shell as you would with a model airplane wing with ribs and covering. Each rib was simply shaped and glued in place. There are no hard and fast rules. The collar area had a thin piece added in an arch to keep track of where the neck opening was going to be placed.
Step 3: Adding the shell covering
Each section of the shell was made by gluing a sheet of cardboard over a single cell of the ribbed structure. The cardboard's corrugation was broken down to get a smooth bend along the ribs. Do this by grasping two ends of the cardboard and running it along the edge of a desk. This will break down the inner corrugation and 'soften' the cardboard. Simply work each section. I usually made the sheet a bit larger than it needed to be and once the hot glue had cooled, I used my razor knife to trim the covering. You can also see the smaller triangle sections added around the collar.