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.