Step 2: Cutting the cardboard
I intended originally to cut my cardboard into one piece that could be folded into the final shape, but I ended up doing it in two parts. I think this worked out to be easier, and I recommend it. All of the dimensions worked for my mirrors and my camera, but you may need to adjust it for your materials and equipment.
Here's a rough diagram of how my carboard pieces were cut out. Depending on your camera and the size of your mirrors, your measurements may be different. Experiment. I've shown dotted lines where I left extra space to cut things down later. Easier to remove cardboard than to have to start all over.