After measuring the dimensions of the vivarium I cut the poly sheeting using the 1ft-10" as height. When cutting the length to size, remembering to subtract twice the thickness from the overall length as I was making sides as well as a back. So my 47" internal measurement went to 43", which allowed me the 2" that each side would take up. The sides were cut to the internal front to back dimension, which worked out to be 1ft-11". So I ended up with 3 pieces 43" x 1ft-10" and 2 at 1ft-11" x 1ft-10" (if you where just making a back only then use the full internal dimension in this case it would be 47")

I had a piece of poly printer packing which was 'L' shaped 1" thick and I used one side to create the hide and another to help make the ramp (if you don't have this use the sheet of 1" poly to make this shape)

Mark the outline of the hide onto the back poly. I used a pencil mark both the outside and inside of the poly, then cut a grove using a craft knife along the marks so that the hide fits snugly and the poly will hold it.

Once you have your basic hide sorted, it is time to make some basic shapes and ramps.

A ramp can be made from some of your leftover 1" and 2" poly. Cut a 2" x 2" piece of poly approximately 10" long, then cut a piece of 1" poly 12" x 7 ½" stick the 2" x 2" to the inch poly ½" in from the edge. The ½" will be used to attach the ramp to the background by once again by placing it on the polly marking the shape and cutting a groove.