Step 1: Start
Toilet roll or baking paper cardboard tubes
1 x carboard box
Roll of string (wool is fine) OR sticky tape (lots)
In this instructable, I will give you an idea you can use for your own cat carry box. A window that your cat can get fresh air and see well through.
First thing is to grab that box and cut a rectangle/square out of the side. This will be where you want the window to be. Make sure you keep the peice you cut out intact in case you need it for something later on - an add-on, maybe.
Step 2: The fun part
Once you have a pile of the rings, start cutting peices of string about 15cm long. You will need to double the sting up and tie each ring to the next to form a grid/mesh. They will all be linked together by the string or if you decided to use sticky tape, you tie them together by tape. I have tied them horizontal and verticle to each other. If you have a big strong cat, you may want to tie them diagonal to each other as well to make the mesh stronger. I can imaging the mesh will be smaller, though.
Use the peice you cut off the window to size up your mesh and to make sure it will fit the window. Once you have a good sized mesh, you will need to make it fit the sides of the window/box to keep it in place.